Thanks to all for the kudos. Here are some answers to some of the posted questions:
They're manatees (note the rounded tail fluke)
The posted photos were taken with a Nikon D300, Tokina 12-24mm with a +2 diopter, Ike housing. I used aperture priority at f5.6, ISO 400, zero exposure compensation. Shutter speeds usually turned out to be in the 1/300 range.
The quality of photos from this area has little to do with the camera system used and everything to do with water clarity and conditions. Your photos will be much better if you can find animals in the head springs instead of in the rivers. All of the posted photos were taken within the head springs at 3 Sisters off Crystal River. The manatees will generally enter the head spring at high tide, and it's also important to go when the area is not swarmed with other people. So, I go in mid-week on a day with high tide early to mid-morning. Even in the clearest waer, it's important to photograph with the sun directly behind you--side or back lighting will really light up the junk in the water. I drive to the area (about 11 hours from Baton Rouge) and pull my own boat, so I can pick the day at the last minute and I have complete control of when I'm on site. Hope this helps. -Clay