If you go north of the bridge at york town, into the cave, you'll find a waterfall west of the draisine rail(note: you do not need the draisine to obtain the flippers), go behind that, and you'll see something swimming around, get your fish out and show it.

With the goggles, you do need the draisine though. You can find various guides if you look around on these forums as to how to go about obtaining them.