This is a very long and tedious achievement. At best, it'll take you a solid hour of "submarining" to get. Here are the main pointers:
1. You need to buy the Sonar Collections Dock ($250,000) on the North West side of the map to get hold of the submarine. Any character can buy it, and any character can get involved with finding some of the nuclear waste.
2. You DO need the submarine. Some of the waste is especially deep, I think the deepest one I picked up was around 500, 550ft down. En route to collecting two barrels, I got a warning message that my sub would get crushed if I went much deeper.
3. Look after your submarine. If you get half way round the island, and then it breaks because you've been driving it like a muppet, or beach it (I did),it's a long way back to land, and a long drive to pick up another sub.
4. There are 30 pieces to find, and they are spread all the way around
the island of San Andreas, so this a whole round trip of SA.
5. Don't pay attention to the numbers on the Trackify app, as they're irrelevant and inaccurate. At one point or another, the numbers were going down, then shooting up again, as if the barrels were running away from me. Only pay attention to the red dot, that is the guide for the barrels.EDIT:
Some people are disputing point five of my solution, saying the numbers on Trackify represent depth. Whilst this may be true, it didn't accurately tell me
depth. At first, I thought it was depth, but when I traveled between barrels constantly on the surface, and the numbers still fluctuated between 40m - 100m, I didn't know what it was doing, and hence, disregarded the numbers. Apologies for any confusion on this point, but I'm just sharing what happened with me.
1. It's a good idea to unlock the side mission for finding the 30 submarine pieces before attempting this achievement, as you can kill two birds with one stone. This is a Strangers and Freaks mission on Michael's side, a blue ? right on top of the Sonar Collections Dock.
EDIT: Link to map of Submarine Parts http://s1166.photobucket.com/user/iKushtyy/media/Submarine-P...
2. Get some music playing to ease the boredom, as you can't get in-game music in the sub.
3. EDIT: A video link showing barrel locations has been submitted to me, courtesy and thanks go to MAKA91.