Jeff,
Sanibel is a great place for boating. We have a place on Boca Grande, just north of sanibel.
In the Gulf of Mexico, you can run along the beaches, just watch for sandbars - especially near the passes (inlets) like redfish, captiva, and boca grande pass.
If you run north along the beaches you will come to Cayo Costa State Park. Its an island and thus there are lots of areas where you can have the beach all to yourself! The shelling is great and our black lab really enjoys the beach ("no dogs" but i've never seen a ranger...) The south end of Cayo Costa is an unofficial nude beach.
The north end of the island has a lagoon with a small inlet (10 feet wide) with a strong current running between the gulf and the lagoon. The kids really like rafting through the lagoon and inlet.
Cabbage Key Inn, on cabbage key, just inside of Cayo Costa is a great little bar/restaurant that serves good hambergers. The walls are covered with $1 dollar bills from past customers, its a step back in time to the old florida and is a must see if your in the area.
Fishing in the passes and on the back country flats and oyster bars is great. I never go over there w/o my boat - have a great time!