I used to live in Treasure Island, soze (it's about 45 minutes to an hour west from Tampa, and lies just to the north of St. Pete) and it has a wonderful HUGE beach (large enough for dozens of volleyball games at once and plenty of room left over).
Even this picture of the beach doesn't do its size justice. I really couldn't tell you much about water temperature in April, but it will still be pleasant enought to stroll a beach that's just too large to get filled up with people.
Go just five minutes north of the public access to the Treasure Island beach to
John's Pass, a cut from the harbor to the gulf that separates TI from Madiera Beach, and sit outside and watch the drawbridge go up and down as the sailboats and motorboats traverse.