I've done a couple of base boards for Dystopian Wars and, more recently,
my Port of Bruegelburg project.
This is a shot of the "harbour" variant:

This is a shot of the "open seas" variant for Dystopian Wars:

Both are made from 3mm MDF boards painted using a paint roller and various hues of acrylic paint, with the port boards using more green and the open seas more blue. On top of this, I added a layer of self-adhesive clear foil which is rather robust. I also like the way objects reflect in the "water".
Obviously, if you could get a 3d version that has some wave pattern on it, it would look more convincing as an ocean surface; still, for small ships, it can lead to unsightly gaps between the ship bottom and the wave valleys as the model sits on the wave crests.
Personally, I opted for a smooth surface and painted in waves and crests. Generally, though, I think it works rather satisfactorily and is a much faster method (and less prone to warping the boards) than applying numerous layers of gloss varnish.