I came across the images shown below, generated with the commercial software PhoenixFD and 3D Studio, and started to explore if I could achieve something similar in Blender.
Using Blender’s Ocean Modifier, I can construct the water surface, but I find it difficult to mix in the HDRI sky. As you can see from my render, I can’t get rid of the black gap coming from the HDRI.
This is a strange HDRI… usually they go all the around in every direction you could of course just move your plane up, (G > Z), but I wonder if you could find a better HDRI somewhere like HDRIhaven.com?
You can easily adjust the horizon line in an image editor. Simply select the sky and scale it vertically. It’s magic! Be sure and save with the same color depth and file format as the original!
But Sterling is spot on point. Your ocean plane must completely fill everything below the horizon line.
To reduce the computational weight you can swap out the far distance with a crude approximation of your near-camera texture. Use atmospheric haze or depth blur to obscure the blend between the two.
you could of course just move your plane up, (G > Z), but I wonder if you could find a better HDRI somewhere like 
