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Security rules prohibit casual visitors entering a cruise ship while in port, for same security reasons that keep folks from wandering into an aircraft parked at an airport. But as always there are exceptions ....
Travel agents can pre-arrange to inspect a ship when it is in port, but this comes with strings attached, such as the volume of business your agency has sent to that cruise line, being escorted around the ship by a crew member, restrictions as to which ports this is possible, etc
Some cruise lines and/or ports allow family members onboard during initial boarding at the port of embarkation. But not usually at stops during the cruise.