There are a total of seven submarines located in the sea. The following is a list of all seven and what you receive for completing them:

Submarine 1: (3,A) Treasure Map 14
Submarine 2: (7,A) Heart Container
Submarine 3: (6,C) Bottle
Submarine 4: (4,D) Heart Container
Submarine 5: (7,F) Red Map, Watch Tower Locations
Submarine 6: (3,G) Treasure Map 22
Submarine 7: (5,G) Treasure Map 9