Why do crabs sink while fish float (if they want to) in water?

Question

My six-year-old son is very curious and really wants a great scientific explanation for this. He's a budding scientist, so the more detailed an answer we can get the better!

Answer 1

because they eat the waste from the fishes .The fishes waste drop to the bottom and thats where they eat off the ocean floor for their food because they don't swim.

Answer 2

crabs do not have a swim bladder like fish do  the swim  bladder helps the fish control its buoyancy and allow it to float in the water