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!