The only thing you need to make appointment times for is the Dolphin Experience and that sea-floor walk thing (I did the dolphins but not the other). The rest of the day is yours to spend as you wish. There are two areas for snorkeling - the Grand Reef which has a colder water temperature and lots of fish, rays etc and the Freshwater Oasis, which is significantly warmer but doesn't have fish because the water is too warm for them. However, it does have a lazy river with 'artifacts' underwater (canons, statues etc) as well as the Marmosets (they're on an island you have to swim up to) and the Otters.
I got there pretty early so my dolphin experience was first thing, which was great because it meant I could just do my own thing for the rest of the day without worrying what time I had to be anywhere. So if you can get there early I definitely recommend it.
Oh and yes, it's completely man made.