1. 물고기: fish (noun). The most common used vocabulary for fish follwed by 생선. xD
2. 고기: fish (noun). It's fish as food. This word is also used when you go out for fishing or when you say you catch a fish. (고기 잡다)
3. 마리: This is for counting fishes. like, 물고기 1마리, 물고기 2마리, 물고기 3마리, ...
4. 낚다: fish (verb)
5. 어류: 魚類 (漢字語). noun fish that refer to a group of animals. It's a vocabulary for scientific classification.
6. 물고기자리: Pisces. a constellation of the zodiac.
7. 생선: 生鮮 (漢字語). Fish as food.