Hello renderboy,
What you are looking for is to have a Post Process volume bound to the surface of your waters edge. I wrote a tutorial for the Wiki that can show you how this is done and to also add a bit of a blur effect as well to increase the realism a bit.
Swimmable Water Volume Tutorial:
Your outcome underwater will look something like this
Let me know if you found this helpful, or if you still need some assistance.
Cheers,