It's not too crowded yet as most schools are still in session, but as the PP said, St. Pete Beach will be less crowded than Clearwater Beach, which is more touristy. We live here and our favorite beach spot is Pass-A-Grille. It's quiet, relaxing, and you'll find on-street metered parking, clean bathrooms right across from the beach, and nice outdoor-type showers for cleaning up before getting in your car. There are restaurants close by, and you can go up the road to St. Pete Beach for more places to eat and shop. You will pass at least one Publix on your way and can always pack a picnic (remember, no glass and no alcohol on the beach). The Don Cesar is at Pass-A-Grille if you want something more fancy. I don't know their rules about using the beach cabanas for a day, but I absolutely love The Don, so if you want a special treat, that's a great resort!
In St. Pete Beach, the Sirata, Tradewinds, and Grand Plaza - great hotels! Tradewinds has a giant slide and cabanas for rent, but I don't know their policy for non-resort guests. We've parked at the Grand Plaza and ate at the beach bar and gone on the beach, no problem, but I don't know if they'd question a whole family going through with beach gear?
About 5 miles from Pass-A-Grille is Treasure Island. We like Treasure Island, but it can be harder to find parking. John's Pass is there and it has tons of restaurants and shops that tend toward the tourists. They also do fishing excursions and, I think, dolphin excursions from John's Pass.
The one thing I notice between the Atlantic and the Gulf is that the Gulf water is a lot less clear. If you snorkel, you will have trouble seeing anything in the Gulf water. As I said, we live here, and have tried to snorkel a few times and it's just pretty murky. And the sand is different. Can't really explain it, it's just sort of "smaller" sand and it gets everywhere, whereas when DH and I go down to Miami and Ft. Lauderdale the sand is "bigger" and easier to get off
Have a great trip!!