The only correct answer to your question is the one by question_asker ^. The idiom "drinks like a fish" is used to describe someone who drinks alcohol to excess. It has nothing to do with real fish, water, breathing or gills. it's similar to the idiom "eats like a bird," which describes someone who eats very little. That idiom has nothing to do with real birds, who actually eat quite a lot. Idioms are interesting! Are you learning English?