I created a simple ocean material with tad of foam on top. You can see that the color ramp is almost entirely green, but there is a small portion of white too.
In viewport the surface of the ocean looks fine, but in rendered version it’s all white.
Hi, you can try changing texture coordinate from generated to object.
Thanks, you live and learn 
Never really thought about the difference between the coordinate types.
Now that I have changed to an ocean floor generated from a cube, I find that “generated” coordinates work better. My previous model was just displaced with noise texture.