:)1.watch the movie or go to the movies? Both are grammatically correct, but there is a difference in the meaning.
If you WATCH THE MOVIE, you can be at home and watch it on your TV. It you GO TO THE MOVIES, you go to the cinema.
2.waters the plants or water the flowers? Both are OK, but you don't need -s suffix after WATER
3.fishing in the sea or at the sea? Fishing IN the sea