Ft. Desoto or leave it at home.
South from the dolphin to Bayway East/West (Look up Don Cesar, that's the turn). Go east ($0.75 toll) to Bayway N/S. Go south all the way to Ft. Desoto ($0.45 toll). You can turn onto the beach just after the toll and launch for free (idle speed out of the flats) or go over the bridge to the 1st right turn in to the boat area. Boat ramp parking is $6 with a trailer and has a fresh water flush station. The boat ramp is directly across from the beach I mentioned, but being unfamiliar with the area, I'd suggest just paying the $6. Approx driving time is 20 mins or less.
At either launch, you are in Bunson's (sp?) pass. Go west (stay out of the grass flats) from the launch to get to the gulf. Just before entering the gulf, to the south is Ft. Desoto North Beach and to the North is Shell Island. Shell island is only accessable by boat so you have full access to the gulf side of the island (don't go back behind the island, it sucks back there). To the west of North beach is a sand bar that can have some ok surf, but the surf will most likely be flat. Most of Ft. Desoto it's ok to go to shore with a boat if it's not a busy weekend (lots of people out they will run you off, but you can go to shell island).
There are other options but I wouldn't recommend them unless you are riding a couch. That other launch mentioned in a previous post wont put you in good riding. It would be intercostal and no beach. All of St. Pete Beach is swim zone only. So if it's a stand up, it's Ft. Desoto and that's about it.
The other option would be Veteran's Park (by Baypines VA), which you take west out to Johns pass. That'd be about 30-40 mins drive and it's about 20 mins from the launch to the beach (lots of idle speed). Then there's lake seminole in Seminole (about 40 min drive) and Dunedin's Causeway (about 1 hr, where most of the Duck Pond guys ride).
Like I said, your best bet is Ft. Desoto ramp, ride the sand bar just off North Beach and hang out on Shell island (they're like 200' apart).
If you make it down this way with the skis, you can contact me close to that time and I may be able to go with you. I don't care if it's flat.
EDIT: If you use the boat ramp across from the Don Cesar (not sure exactly where it is, have never used it), it will put you in the intercostal. A short ride south will bring you to the point of Pass-a-grill, which is directly across from the north end of Shell island. You can ride and hang out anywhere on Shell island, but don't go north to St. Pete Beach. So I stand corrected, there is another option other than Ft. Desoto ramp.