I've found a cool, yet simple exploit that allows CJ to fly the jetpack underwater and walk on the sea bed, without any cheats. It's great for collecting oysters.
1. Play through the storyline until you complete the mission "Green Goo". This will make the jetpack available.
2. Find a Blue Hell entrance: http://gta.wikia.com/Blue_Hell
Scroll down to the Entrances/GTA San Andreas.
Or you can try an entrance that isn't mentioned there, but is confirmed by me. It's located in south-western Las Venturas, a few meters to the west of an overpass which runs above the Julius Thruway South. It's on the northern side of the highway. The Blackfield Chapel is to the south of that spot, across the highway. It may take a few tries to fall through the hole, but it works.
3.Take the jetpack and descend into Blue Hell using the entrance that you chose. Fly to Las Venturas(if you aren't already there) and head to the eastern coast of the city. Go under the barren hills on the coast and continue flying to the east until you are below the flat sea bed(outside the world boundaries). This sea bed is not solid, so rise through it. Now you are no longer in Blue Hell; in fact, you're flying the jet pack under water. You can fly to solid sea bed(within the world boundaries) and land on it. Now you're walking on the sea bed.
You can easily explore the depths of any contiguous body of water in San Andreas this way. Since you have a jetpack, you can fly anywhere you want and get the oysters with little effort. Of course, if you resurface, you'll have to do the trick again.