North of Clearwater, called the "Treasure Coast", you will find a lot of rivers but not much beach. We love to kayak, and this is our favorite area to camp.
One favorite of ours, about 1/2 hours north of Tampa in Homosassa on the Homosassa River, is Nature's Resort. We will kayak the Homosassa River up to the head springs where you can swim and see manatee (you could also visit the State Park, but not via kayak), and then down to a little dive called the Manatee Pub. http://www.naturesresortfla.com/
Down at the very southern tip of Pinellas County is Fort De Soto. https://public.co.pinellas.fl.us/parks/ParksMain.jsp
Excellent for hiking, visiting the beach, snorkeling, kayaking. You can visit the old fort. You can also take a ferry over to Egmont Key for more snorkeling, gorgeous beach, and another fort.
Then farther south is Koreshan State Park on the Estero River for more kayaking and hiking. Welcome to Florida State Parks
The only place in the Keys that we've camped is Curry Hammock State Park. LOVED it and highly recommend it. Very small campground, but great location.
Next we want to try Long Key State Park.
I have heard great things about Bluewater Key (like an RV timeshare and you can rent sites from owners), but never stayed.