I think semantically that would be correct, but because a sea is a very large body of water, you'd almost exclusively be talking about one specific sea ("the sea").
Yes, the sentence may seem unusual, but I think it depends of the context: if you are orbiting in a spaceship around the planet and, suddenly, a blue spot appears in the middle of a continent, you could affirm "I see a sea". Do you know what I mean?
Lol well good example given your username there. I don't disagree with you at all, though.
That would be "смотреть на море". It is similar to the difference between hearing someone and listening to them.