You should check out Christoph's water tutorial:
http://www-public.tu-bs.de:8080/~y0013390/pov/water/
Specifically, Part 4, which talks about the interior of the water. When your
water is an opaque surface, it will end abruptly at the coast. When the
surface is transparent and you rely on the interior to block out the ground,
shallow water becomes clear while deep water remains murky.
- Slime
[ http://www.slimeland.com/ ]
