ST. PETERSBURG — The pilot of a small plane died when he crashed into the water south of Albert Whitted Airport moments after takeoff Sunday afternoon, the U.S. Coast Guard said.St. Petersburg Fire Rescue divers helped extract the pilot, 70-year-old Donald Thomasson, who was still strapped into the cockpit seat when they reached him. Witnesses reported the crash of the single-seat aircraft to authorities just before 3 p.m. after it nose-dived less than a mile from the airport near downtown St. Petersburg. St. Petersburg police said the plane, a Piper PA-23, belonged to Aerial Banners Incorporated, a company that tows advertisement signs and is based at the airport. Company representatives from Fort Lauderdale traveled to the scene Sunday. According to witnesses who talked to police, the pilot had just picked up a banner and was beginning to climb when they heard a popping noise before the plane crashed nose down into the water.The plane landed about 75 yards south of the seawall at Albert Whitted. Rescuers extracted the pilot, who was dead, and transported his body to the U.S. Coast Guard station near the airport.Several witnesses told the Tampa Bay Times that, from where they were gathered in Lassing Park south of downtown, they could see the plane take off towing a banner. It wasn't in the air long, they said.As the small craft climbed, Gene Alston, 44, said the pilot released the banner it was dragging before he could even read what it said. Seconds later, the plane nose-dived, turned slightly and crashed into the waters below, not far from the runway. "It went so fast," said Lenora Jankovic, 50, who was with Alston when the plane went down.Amanda Clark, 18, said the plane sank within seconds, so fast that she and her friends couldn't get their phones out in time to take photos. After the bay waters had swallowed the craft, the group said they heard a bang, like the plane had exploded underwater. Nearby, Carstell Holloway, 48, said it "sounded just like a boom, pow! I thought it was a boat hitting some rocks."Holloway said the plane sounded like it lost power after taking off. "Usually you can hear that plane make a racket when it takes off," he said. "This time it went quiet and then it crashed."St. Petersburg resident Gary Smith was casting for redfish a couple of hundred yards off of the park when he saw the plane take flight. He thought it took an "abnormally steep" climb when it left the runway. He didn't think much of it and turned back to fishing. Seconds later, he heard the plane crash. "It sounded like someone putting a cigarette out in the water," said Smith, shirtless and standing chest deep in Tampa Bay. Several boats converged within seconds on the area where the plane went down, witnesses said, as if they were trying to help rescue the pilot. Then a loud siren sounded from the marina and rescue boats began responding. From their spot in the park, those gathered said they could see fire trucks and ambulances on the runway. Alston said at least a dozen boats floated around the area where the plane crashed. A Coast Guard boat arrived on scene, police said, about six minutes after the crash, followed by a St. Petersburg police boat. Though the Coast Guard station sits right beside the airport, they did not have a dive team to send to the scene, Coast Guard Petty Officer 3rd Class Ashley Johnson said, so Fire Rescue divers responded instead.Florida Fish and Wildlife Conservation Commission and the Pinellas Sheriff's Office crews also were on scene to assist in the rescue effort. The plane was still submerged in about 25 feet of water late Sunday, police said, and likely will be retrieved sometime today. Federal Aviation Administration officials were on scene to investigate. "It's a real shame," Smith, the fisherman, said. "Such a beautiful day for such a thing to happen. Real shame." Times staff writers Graham Brink and Laura C. Morel contributed to this report.