Upvote:6
No ferries, much too far. Best you could do is a cruise ship, but the only ones that tend to do HK - Yokohama are the ultra-luxury ocean liners like the Queen Elizabeth, and then only once a year.
If you have a lot of time there are some cargo lines that accept passengers. Cargo ship schedules are very flexible, you have to be ready to go when they say so.
There is a ferry from Busan, Korea to Fukuoka, Japan but you have to get to Korea first and then travel to the coast so it will probably cost the same overall.
US$700 return is not bad, a good seat sale might get it down to $500.
However, given the daily cost of living in Japan, can you afford the rest of the trip? It's not somewhere you go for a cheap holiday, that's Thailand.
Upvote:6
The various Asian LCC (low-cost-carriers) airplane companies are the way to go. Check out Air Asia, Peach, Vanilla Air, etc. You should be able to travel for way less than US$300 return if you book at the right time.
For example, this flight from Hong Kong to Osaka one way is less than US$120 on Peach:
Similarly, this flight from Taipei to Tokyo one way is less than US$120: