This collection mission only becomes available if you purchase the "Sonar Collections Dock" south of Paleto Bay. The purpose of this property is to clean up the oceans around San Andreas of nuclear waste.
The property itself costs $250,000, and is only available after completing Main Mission #35 (The Merryweather Heist), as that is when the sub becomes available. However unlike most optional missions and hobbies in Grand Theft Auto V, you won't be collecting Nuclear Waste for charity. Instead you will be paid $23,000 for each piece you find.
Quick math says that you'll need to find 11 pieces in order to make this property worth it, however you should also know that there are 30 pieces of Nuclear Waste in the game, meaning you can make a total of $690,000 by collecting all of them (for a net total of $440,000).
Finding these things isn't easy though: you will need to use the dock's submarine and go out and find them in the ocean (all of them are in the ocean, in case you were wondering). To assist you (since the ocean is so fricking big), you are given a phone app called "Trackify".
Trackify shows you where the nearest piece of Nuclear Waste is by radar, and then also tells you how far down it is in meters. You will need to head to the piece of Nuclear Waste and then dive down until you find it.
Since all of thse are underwater, writing down the locations for them is really not very viable. Instead, refer to our video support link below to find them all!