If you pay attention, you will see the same thing happen multiple times per trip. In the past, it could've been a chinese catfish (puntat), or a snakehead (pongee), but those fish are rare nowadays. I am 100% sure that what you saw was an armored catfish. In the aquarium trade they're known as plecostomus. They are an introduced species that inhabit ponds and rivers all over the island. Some here feel that they are eating the algae and other plant matter that the shad need to survive.
Here's one I accidentally snagged on a lipless crankbait. They get much bigger than this. Look along the bank when the water is low. All those holes were made by these fish.