What arguments do adherents of a global flood give for how water was able to cover the entire earth?

Orthodox Christian monk and former philosopher Seraphim Rose addressed this question in his book, Genesis, Creation, and Early Man. Abbot Damascene, who edited the 2nd edition of the book, comments (p. 408):

That the Flood was universal is witnessed by the vast extent of sedimentary deposits (formed by aqueous action) over what is today dry land. O.D. von Engeln and Kenneth E. Caster, in their book Geology (p. 129), write: "About three-fourths, perhaps more, of the land area of the earth, 55 million square miles, has sedimentary rock as the bedrock at the surface or directly under the cover of mantle-rock ... The thickness of the stratified rocks ranges from a few feet to 40,000 feet or more at any one place ... The vast bulk of the stratified rocks is composed of shallow water deposits."

Monk Seraphim himself writes:

The questions of what mountains were before the Flood and how high the Flood rose cannot be solved conclusively, because it is very likely the cataclysm was so extreme that the whole earth was quite different then. All the mountains may have risen up at that time. Entirely different geographical and geological features would have been created. Those people who accept the idea of the Flood in their studies of geology - like Henry Morris, in The Genesis Flood - say that most of the layers were formed during and right after the time of the Flood, not over millions of years. Read the book.

I would also add that the fact that native peoples from every continent in the world (except Antarctica) have some sort of mythological recollection of a great flood having occurred in their region strongly suggests that the Flood, if it did occur, was global and not local.

Onr popular theory is Dr Walt Brown's Hydroplate Theory.

The basic premise of the theory is that most of the land was gathered together at one place. There were mountains, but perhaps 3,000 or 7,000 feet tall. At this altitude, there would be more than enough water to cover the entire Earth, considering the water both beneath the crust of the Earth and in the atmosphere.

When the flood occurred, and the "fountains of the deep" were opened, the mid-Atlantic ridge ripped open "like the seam of a baseball", and the water beneath the plate shot upward at supersonic speed. As the land eroded quickly at the seam, the mantle beneath began to bulge upwards, creating the mid-Atlantic ridge.

The plates, lubricated by the water beneath them, slid (the whole continent), reaching speads upwards of 45 miles per hour. When they reached near their current position, they encountered resistance, and the plates buckled, forcing them upwards in places such as the Rocky Mountains on America's West coast.

Dr. Walt Brown goes into much more detail on his site, with the full contents of his online book available to read or obtain a hard-copy.

The positives of his approach is that it systematically explains a very large number of natural phenomenea particular to our present Earth, including the fossil record, the different strata, the mid-Atlantic ridge, the mountain ranges, volcanos, asteroids and comets, and several high plateaus (including the Colorado Plateau), as well as many other features. He details his findings quite well, coming from a military career with degrees in Mechanical Engineering to back up his findings. He presents a strong case for a global flood.

I think discounting underground water reserves is folly. I work in the oil/gas industry testing recovered hydrocarbons (oils, gases, condensates) and various fluids mixtures (natural waters, pumped KCL water, pumped frac oils, pumped acids and other various pumped chemicals for viscosity or inhibition) for content, quality, and velocity, or how much force on surface the reserve has. Usually we deal only with 5,000-10,000 psi wells, but there occasionally are wells with greater pressures on surface, and I've seen some of the bottom-hole pressures at upwards of 70,000 psi on the frac crew equipment) after a gas or oil well has been drilled (However, sometimes they contain just water [called dud wells, as are ones that turn out incredibly lower pressures or content than predicted, and consequently are an oil companies nightmare considering the huge costs to explore, prep, dig, complete, and then maybe abandon the well]). We can only (fairly) accurately see with seismic to about a depth of 7 kilometers. I worked on an exploration well where we went to that depth drilling. It was the only well I've personally worked on that was nearly over 15,000 psi working pressure. A very hairy job, watching 3 inch thick iron pipe [many thousands of pounds of weight] jump around on the ground like it was going to pull itself apart while we flowed it back). I've read of experimental holes dug for scientific purpose up to about twice that depth, so around 14 kilometers.

To put that into perspective:

"Earth's mantle extends to a depth of 2,890 km, making it the thickest layer of Earth."

We can visually "see" what is underground using seismic equipment to about 7km, that gives us a fairly accurate picture for 7k of 2,890km or 0.0024% of the the mantle depth.

Also, we have gone to about 14km of 2,890km, or 0.0048% of the depth.

Seems like hubris, or just plain ignorance, to automatically assume we have a good grasp of just how much water is in there. There may still be plenty of pockets, or aquifers yet undiscovered.

Also keep in mind things react differently, even unexpectedly, under some of the extreme underground pressures. We see this all the time in flowing wells. And water underground begin squashed down by tens of thousands of pounds-per-square-inch of pressure isn't the same as water sitting on the surface at atmospheric pressure, first thing it does when it hits surface if mixed with the right combinations of gas/oil is turn to tons of foam. We have to pump tons of inhibitors and/or diesel (diesel helps cut foam and is easier/cheaper than foam inhibitors) to keep from blowing it out the flare-stacks, down the pipeline, our out the 400bbls. It's a real pain, especially when the frac process pumps CO2 to fracture a well (think shaking a can of soda, heating it to 60 degress celcius [140 degrees for Americans], then opening it fast).

Finally, there are also strange occurrences that just go against the norm when dealing with the depths of the earth. We work through it and get the gas/oil, but even the engineers get stumped by some of the ways the wells behave, some of the odd mixtures of fluids/gasses we recover, or by the science going wrong and giving us dud wells when everything pointed to real 'screamers'.

Anybody putting their faith strictly in science is destined to be let down, that's the only constant scientific fact.

In the end, scientists are just people, and science is just ideas people have based on the current information available, and later on others will change those ideas by new information that comes along, and proves the earlier scientists were either dead wrong, or just didn't have another piece of what will always remain an incomplete and infinite puzzle.

1. The current estimate of the amount of water on the surface of the earth is about 1.3 to 1.5 billion cubic kilo-meters http://hypertextbook.com/facts/2001/SyedQadri.shtml
2. We recently found out there may be up to 3 times more water under the earth https://www.theguardian.com/science/2014/jun/13/earth-may-have-underground-ocean-three-times-that-on-surface
3. This leaves us with a possible (again we are dealing with very theoretical numbers) 5.2 - 6 billion cubic kilo-meters of possible water.

How much water do we need to cover the surface if the current earth?

1. THIS IS NOT MY MATH, took this from a forum--if someone has a better number pass it on A good approximation for the radius of the Earth at sea level is 3960 miles. VEarth at sea level=4/3 pi r^3 = 260,120,252,602.5 cubic miles, approx. Everest is about 5.5 miles above sea level, so we add that to the average sea level for a radius of 3965.5 miles, approximately- VEarth completely flooded=4/3 pi r^3 = ~261,205,593,010.8 cubic miles. Subtracting the first volume from the second, we get 1,085,340,408.3 cubic miles.
2. 1,085,340,408.3 cubic miles to cubic Kilo-meters = 4523896164.29169 cubic Kilo-meters

An estimated 4.5 billion cubic Kilo-meters more water from sea level is needed, we have an estimated 5.2 - 6 billion cubic kilo-meters total water available.

Current water (1.3 to 1.5) + water needed (4.5) = We need a total of 5.8 to 6 billion cubic kilo-meters of water to flood the earth.

We have 5.2 to to 6 billion cubic kilo-meters of water available

Superduper theoretically... we have enough water on and in the earth and don't need to invoke pseudoscience.

Was there enough water in the world to cover the earth?

The ancient geology according to the Bible is rather interesting.

There was enough of an elevation differential for rivers to flow.

Genesis 2:10 And a river went out of Eden to water the garden; and from thence it was parted, and became into four heads.

Rain was not needed.

Genesis 2:6 But there went up a mist from the earth, and watered the whole face of the ground.

There were "mountains"

Genesis 7:20 Fifteen cubits upward did the waters prevail; and the mountains were covered.

The flood itself involved waters "from the deep" as well as "the heavens".

Genesis 7:11 In the six hundredth year of Noah's life, in the second month, the seventeenth day of the month, the same day were all the fountains of the great deep broken up, and the windows of heaven were opened.

Some have suggested that the "fountains of the deep" were released via a rupture of the Atlantic seafloor. This water under the pressure of the overlaying seabed would jet very high into the atmosphere if it were released at one time. This could account for how mammoths were flash frozen with tropical flowers still in their stomachs.

Such an upheaval on the surface of the earth could have resulted mountains raised high enough so that they were much higher than they were previously.

Gen. 8:2-5 talks about the fountains of the deep and the windows of the heavens closing, and the waters receding and diminishing anyway. You would expect a natural flood to either drain into the earth—but the "fountains of the deep" were closed, so the water could not leave that way—or dissipate by evaporation—but the "windows of the heavens" were closed, so it couldn't leave that way, either.

The text seems to require pairing the miraculous appearance of water (the Flood) with the miraculous disappearance of the same water afterwards. In this case the question of whether there is enough water in the world now to cover it is moot.