Try Koloa Landing, near Poipu.  It's a small boat launch at the mouth of a
creek emptying onto the south side of the island. A little west of the
Sheraton.  It's a nice shallow dive, with lots of humuhumunukunukuapuaa (the
state fish--a Picasso triggerfish that's mostly found near shore).  I saw
many eagle rays there too.  It's an excellent snorkeling site as well.  If
you have a kayak, you could go out to Sheraton Caves, which has lava "tubes"
(which aren't usually tubes--more like half pipes) and lots of turtles. But
then, EVERYPLACE on Kauai had lots of turtles.

If you see turtles swimming toward a rock outcropping or underwater mesa,
watch how the butterflyfish wait for them at the edge and then swim up and
eat the algae off their shells as they swim across. When the turtle reaches
the other side of the mesa, the butterflyfish drop back off and wait for the
next turtle.

http://www.shorediving.com/Earth/Hawaii/Kauai/index.htm