If you light all 5 red lights above the right flipper, the ramp to the Treasure Chest diverts to the Sunken Boat. If you leave the Sunken Boat by the hole at the bottom of the area, you are awarded a bonus based on how many times you hit the pop bumpers (1000 points each time) times a multiplier of 5 if none of the lanes is lit, 10 if one is lit, 15 if two are lit, and 20 if all three lanes are lit. In Wild West (another Gameprom game), if you pop out the top instead of going through the bottom you get a multiplier of 1 times the pop bumper value instead of losing everything, but it is not known if that is the case with The Deep. The position of the lights on the lanes can be changed by hitting the flippers, making it easier to keep the lights lit.
If there is more than one ball in play, the ramp does not close until the ball in the Sunken Boat leaves, so you can knock the other ball in there too. As long as one ball stays in the Sunken Boat, you can keep it open indefinitely.