THE PLT STATED THEY HAD JUST PASSED OVER A SCHOOL OF DOLPHIN WHILE ON A SIGHTSEEING FLT OVER WATER AT ABOUT 100 FT. HE REVERSED COURSE TO THE LEFT IN ORDER TO RETURN TO THE FISH & WHILE IN THE TURN THE ACFT ROLLED ABRUPTLY TO THE LEFT. ACFT ROLLED INTO A VERT BANK & THE ACFT HIT THE WATER INVERTED & SANK IN ABOUT 12 FT OF WATER. THE PLT STATED THAT HE HAD OBSERVED A C-130 TYPE ACFT FLYING IN THE AREA AT APPROX 800 FT JUST BEFORE THE ACCIDENT. THE PLT STATED THAT HE & HIS WIFE RECALLED HE HAD BEEN MAKING A LOW STEEP BANK OVER THE WATER AND HE RECALLED GRAVITY FORCES PRESSING THEM DOWN INTO THE SEAT DURING THE TURN. THEY HAD COMPLETED THE 1ST STEEP TURN AND WERE IN THE 2ND BANK OF ABOUT 90 DEGS WHEN A 'GUST' WAS FELT & THE NEXT THING THEY KNEW, THEY HIT THE WATER UPSIDE DOWN. NEITHER PAX RECALLED SEEING ANY OTHER ACFT IN THE AREA. ONE PAX STATED THAT HE HAD SOME PREVIOUS FLT TIME INCLUDING SOLO.