From my experiments, and this ruins my concepts how this works, the "global" sea that fills the map has to have holes where land meshes are, I thought (and tests proved) that the sea mesh repeats all over, but in NA map in some areas you can see chubks of land hovering ove water with empty holes underneath. Not having the holes in sea mesh causes horrible Z-buffer issues from distance in sea/land transitions.
To me, the more time I spend with it, the old engine with reflective animated water could do just fine at the current new engine's state and info available to us (which is none in fact)
I use GTOPO 30 DEM, has among other formats grayscale heightmap in it included for given part of the globe. I know GTOPO30 isn't state of the art resolution, but for test work should do just fine