They're fine with you bringing your own towel, although they are provided there too. I usually brought one anyway, though.. just in case I wanted it for the walk back out to the car.
Watershoes or wetsuit booties are a good idea, as the 'Tropical River' is full of rocks and somewhat jagged outcroppings. I scuffed my feet up and cut up my toes on more than one occasion there. I found either neoprene socks with my Tevas or my 5mm booties with tread worked pretty nice.
Fins are out. When they first opened, they let you use fins in the Tropical Reef and river. By 2003 they had prohibited them in the reef all together (apparently it frightens the eagle rays.. perhaps they got kicked by a less-than-skill tourist by accident). And I'm pretty sure they don't even let you use them in the river anymore. So don't bother bringing them.
The snorkels they give you, at least last time I was there, were yours to keep (it would probably be pretty unsanitary to reuse them!). But I always brought my own with me, as it had a nicer dry valve at the top. I still kept the ones they gave me (I have a whole bag full of them at home now.. I use them as backups or gifts, ha!)
The masks they loan you are okay. They /are/ actual scuba/snorkel masks and not some
walmart knockoff (by ScubaPro, I believe). But I also prefer my own gear and used my own.
They pretty much won't let you bring any other dive gear in, although I did sneak a small hand light in once. They generally don't like you having anything other than an underwater camera in the reef, though.
I tend think rented / loaner wetsuits are gross, so you might consider wearing a dive skin if you have one and are particularly picky about who's peed-in neoprene you're swimming in.

I didn't have one at the time, but I did see other folks wearing them from time to time.
Nice to see another diver! If you're from out of state, I hope you'll be checking out some of our dive sites here in FL while you're here!
=)