Hi.
What is the difference between Lake and Ocean in the EE?
For example:
I have a very big world where the sea takes only 1/4 part.
What is better to use in this case? Ocean(for the whole map) or lake (for the 1/4 part)
Which variant will have better perfomance?

The main difference is that Ocean will allow you to make water bumps (vertical waves), while Lake is truly flat.
Besides that performance should be similar, with the exception that Lake is smaller and it may get Frustum culled more often, and not rendered.