Drink like a fish
You may think that fish, since they swim around with their mouths open, drink a lot of water. Actually, they don't. Anyway, idioms are not created by biologists. 'To drink like a fish' is to take alcohol regularly.
The phrase is usually used when being critical of someone behind their back: 'I wouldn't trust Tony to do it. He drinks like a fish.' This does not mean that the person is an alcoholic, which is a term used for someone who is addicted to drink.
Fish are common in our lives and there are plenty of idioms containing them. A big fish in a small pond is an important person in a small-scale organisation. Some people find this more comfortable than being a small fish in a big pond.
When someone is in the wrong setting (for example, a fashion-conscious Hong Kong woman on a pig farm), we talk of a fish out of water. You can see these fish in wet markets, flopping around unhappily.
To us warm-blooded mammals, fish seem cold and this gives rise to another idiom. Someone who seems to be unfriendly and without feeling is referred to as a cold fish. (I have never really liked Mr Lee. He seems such a cold fish. I have never seen him smile.)
Finally, the adjective fishy means suspicious. This arises from the smell of fish. That sounds fishy; in other words, it doesn't smell right.