So turning the floor into an ocean is a little strong. Your leviathan's downside is basically he has to take a minute or so breather, which is hardly a downside at all. Also variables such as, "Twice as strong." aren't very helpful if we don't know his base strength. We can assume human, but is he as strong as a body builder human or a baby human? My paladin for example is obviously inhuman, and given he wears heavy armor and he uses a large sword one can assume he is rather strong.