@IllusionSlayer: You can totally destroy them, but the more intact they are the more money you get. Best thing is to hit the right spots on the spine so that they bleed out quick enough but don't just saw through half the fish getting it to die fast.
On the levels with the boats holding a lot of people, sink the boat and watch the people that fall in. Eat those that try to swim away but there will always be 1-4 that just sit and stay under the water without moving. Leave those for when you're out of boats at the end of the level, so you don't waste time just sitting there.