This would likely require a lot of math if you wanted to get really technical. First you would have to find out how many it would take outside of water, then find the weight and mass of the sunken galleon in correlation to how buoyant it would be. However, the simple way to put it is things "weigh less" in water. It is easier to lift something while submerged in water due to the upward force "assisting" you due to the buoyancy of said object. Something very buoyant, like a human, is far lighter in water than on land. Something less buoyant like a heavy metal object would still be far lighter underwater, but not as much so as something as dense as a human-being or something of the like!
So I couldn't find anything in the books or online about how being underwater affects your ability to lift things. Does anyone know how this should be handled?
For reference I was trying to figure out how many water elementals would need to be summoned to lift a sunken galleon (around 300 tons by my research) and there was nothing about how being underwater changes carrying capacity.
If you have a magical effect that removes the water from the inside and keeps it out... the galleon will simply rise to the surface.... until the magic runs out.
The old 3.X Spell Compendium actually had a spell whose specific purpose was to raise sunken ships.
The rules answer is that being submerged doesn't change your weight.