The Devil and Mr. Frog

Just as the ransom money has been delivered, the kidnap victim escapes; one of his two kidnappers falls to his death trying to chase him. The surviving kidnapper concocts another scheme: he holds the ransom money (which is hot) hostage and tries to coax the kidnapped boy's greedy father into laundering it for him. The kidnapper, an accomplished scuba diver (the source of the episode's title) plans to swim underwater to and from the dropoff point. Five-O identifies the kidnapper and tails him, at one point donning frog masks in an attempt to rub his nose in his activities, but the kidnapper relies on the father to shake any tails and deliver the money to him at 25 cents on the dollar, the standard rate for laundered money. It sounds like a great bargain for the father and he holds up his end of the deal -- but that's not the end of it.