Questions & Answers about Кит плавает в море.
This is a very common question.
- плавает usually means swims / is swimming around / swims habitually
- плывёт usually means is swimming right now in one direction
So:
- Кит плавает в море. = a general fact, or a neutral description
- Кит плывёт к берегу. = the whale is swimming toward the shore right now
In a simple example sentence like this, плавает sounds natural because it describes what a whale does in general.
Because в means in here, and when Russian shows location after в, it normally uses the prepositional case.
So:
- в море = in the sea
A useful contrast:
- в море = in the sea
- в море? location
- в море can look the same as the dictionary form here, because море is one of those nouns whose nominative and prepositional singular look identical
So the case has changed grammatically, even though the form happens to stay море.