For offshore reef snorkeling we highly recommend Biscayne National Park (reservations: 305-230-1100). But for viewing sea creatures, and for convenience and price, nothing beats the rock jetty that demarcates the northern edge of Government Cut. You don't need an inflatable life vest. It's great to have a quality snorkel and swim fins, of course, but in truth all you need is a mask. The only other thing you'll pay for is parking in the lot at South Pointe Park, where Washington Avenue ends at Government Cut. Head out on the north (Miami Beach) side of the jetty. The farther you go, the deeper the water and the bigger the fish. You'll encounter virtually all the species you'd find out on the reefs further south, including grunts, butterflies, angels, parrots, eels, barracuda, and more.