The higher the amperage rating the more power you can pull through it without damaging it.

Personally I wouldn't run anything under a 40A in a boat. If you want to really be on the safe side of things run an RC Hydros 90A unit and you'll be 99% bulletproof!

Be carefull of the ratings of car esc's, the numbers they'll tell you are way out of nowhere and are extremely misleading. Stick to a good marine esc and you'll be in good shape.