One of my favorite things to do in Kauai is to swim with sea turtles at Ke'e Beach.  This is my favorite beach by far in Kauai, located at the end of the road on the island's north side.  Parking is hard to come by, so go early in the day.  If you go just beyond the reef, the turtles hang out there.  The challenge is to make it out over the reef.  The water is about 8 inches deeper than an average person floating depth, so you have to basically do the smallest flutter kicks to get out over the reef without touching it.  The work is worth it.  These turtles are awesome!