I can't help with the first part too much, however, for part two, I would say it would have to be below the center of gravity due to many ships having a center of gravity rather high up (where water shouldn't be). This gives ships their somewhat stability in water (stay upright). If the center of boyance is below the center of gravity, then the ship would be able to maintain its position high enough above the water to remain boyant.
HOPE THIS HELPS SOME. Please let me know.