Believe or not there is a hint for this in the game if you ride the little green car around on the lake floor.
Next ride the little green car on the lake floor which can be found by hanging a right at the drain. You will need to listen to the song that the radio on the lake floor wall is playing.
Finally, go back to the radio room and tune the radio to the song that you heard on the lake floor wall. A door to your right should open.