Key points of GTA V Map - Submarine Parts
Property – It is required to buy this property, in order to unlock the submarine pieces.
To unlock Submarine Parts, you have to finish mission "Death on the Sea". This mission is only available for Michael and appears when you buy a property marked on the map.
When you begin it, a boat for finding Submarine Parts will appear in docks. When you get in it, you'll see a green marked area on the map: this is location where you can find Submarine Parts. When you get there, notice a white circle; the lesser the circle, the closer you get to the part. In one location you find, in general, several parts and you'll have to dive to get to them. Don't worry, because this boats gives you diving equipment automatically.