Well because the water is a plane - its means its just a 2D object. So what a lot of games designers do - including me, is have a camera effects as soon so your character goes under the water. Of course that effects will be blurred vision and trickles (of some sort).

So basically you want something that says: If character goes below -20 - then character is in water:

static var InWater == false;

if(transform.position.y < -20)
{
InWater = true;
}

Then with that variable being true you can activate the camera effects with it.

I am not an expert, but try checking the demo project called "Tropical paradise". In this application, you get some sort of faded effect when diving underwater, so maybe it can help you.