We arrived at 10:30am and got the second to last dolphin swim of the day, 3:45pm, so I guess most people (though certainly not all) arrive early.
We stayed in the water right until 5:30pm (closing time) and then hit the showers/change room. They told us that the gift shop and photo pickup doesn't close until the last guest has left, which is good to know. I think by the time we were changed and all it was just after 6pm that we picked up our photos. And there were still lots of people changing!
We definitely had enough time to see and do everything several times, but at the same time could have stayed hours longer... next time we go, I'd definitely be there earlier.
Oh and we did have to have lounge chairs quite far away from the swimming areas because the close ones were all taken (but we were close to the dolphin swim pools, which was nice too, and after walking at disney world everything seems super close here anyway)!