We've chartered out of St. Petersburg a couple times, but my knowledge of the area is very limited. We loved St. Petersburg. It's a beautiful city with the usual amenities (good restaurants, museums, shopping, parks) if you're looking for a change of pace after being in the "boonies."
We enjoyed anchoring off Gulfport on Boca Ciega Bay in the St. Pete area. Gulfport has a lot to offer as well (restaurants, small grocery). It even has a public beach/park with picnic areas, though it's not on the Gulf. It has a nice protected anchorage and dinghy dock.
We also spent a couple nights anchored off Fort Desoto Park on the south side of Tampa Bay near Bradenton. It's a good anchorage, and the park has a nice little walking trail, but there isn't much else there. It's also exposed to all the traffic running in and out of the Manatee River. Personally, I'd pass it by and head further south to the Longboat Key/Sarasota area.