This was a term used by Chief Thunderthud on the Howdy Doody show in the fifties. It was picked up by surfers in southern California in the late fifties, then skateboarders. Not Swahili, not Hawaiin, not Mutant Ninja Turtles - Howdy Doody.
Kowabunga Buffalo Bob! - Howdy Doody Show
Kowabunga! That was an awesome wave!