I'm sure I've "camped" at John Pennekamp more than almost anyone on here - 78 times I've been there - haven't bothered to figure out how many nights. :B
Actually I was there last weekend Fri-Mon
All that being said I don't have much to add to what has been already been written.
One beach (cannon beach) is about a 1/2 mile from the campground. The other beach is over a wooden bridge (RVs are not allowed over it) just under a mile from the campground. Both are an easy walk or bike ride from the campground. As has been pointed out, if you're expecting a nice sandy beach, you're going to be disappointed. The only sand on beaches in the Keys has been trucked in from somewhere.
Canoes and kayaks can be rented (fairly expensive) for use on the numerous waterways through the mangroves. You can also rent 20,22 or 23' center consoles. There is a large glass bottom boat, the Spirit of Pennekamp for trips over some of the reefs, along with 4 40 passenger or so snorkeling boats and 1 boat for dive trips. Campers with boats can leave them in the water at no charge, in one section of dock that's about 90' long and is first come first served.
The water temp in March will probably be in the low 70's. Air temp.- you never know. It could be in the 80s or the 50's.
I believe over the air TV is still near non existent there. I've had satellite TV for many years and it can be a challenge getting a signal through the tall trees at the back of the higher numbered sites. A portable dish comes in handy there.
I'm sure you'll enjoy your stay.