Oysters are located throughout all of San Andreas, and can only be found in water. You receive $100 for every Oyster you find, and when you collect all 50, you will get $100,000 and your lung capacity is maxed. Also, your sex appeal will be maxed out when you gather all the oysters, so you will be able to go out with girlfriends and always be asked in for "coffee" after a successful date. A good vehicle to use in retrieving the Oysters is the Seasparrow, since it can land on water.
NOTE - These are done in sequential order, meaning it can be confusing picking one at random.
LOS SANTOS OYSTERS
The starting point for the oysters in Los Santos will be Playa Del Seville on the southeast side.
#1) Find the pier that sticks out on your end from the beach in Playa Del Seville. The oyster is at the end of pier, in the water.
#2) Go south to the island section of Ocean Docks. On the south side is a dock just to the west of the ship on the easternmost side. The oyster is right next to the docking area.
#3) Swim west from the last one and find the little inlet. Look up and find the bridge that passes over this inlet. The oyster is underneath this bridge.
#4) Swim west again to find the southernmost bridge that connects Ocean Docks to Los Santos International. The oyster is under this bridge.
#5) Get back on land and head to the beach just south of Los Santos International. Follow it west until it starts to curve north. Look at your map and follow the bottom of the road to your left. The oyster is located just off the beach parallel to the bottom of the road.
#6) Swim along the coast north until you get to Verdant Bluffs. Find where the road begins to enter a tunnel. The oyster is located near the beginning of the tunnel near the cliff.
#7) Continue up to Verona Beach and find the two pedestrian bridges just west of the long pier. The oyster is under the north one.
#8) Go west to Santa Maria Beach and find the lighthouse to the left of the long pier. The oyster is just south of the lighthouse.
#9) Go just north of Verona Beach to find Marina. There should be a small river-like waterway a bit to the west of the burger place. Find the northernmost bridge and you'll find the oyster underneath it.
#10) Now, head all the way to Glen Park, to the actual park you should have noticed in a few missions involving gang territories. You should find the oyster in the pond in the middle of the park, underneath the overpass.
#11) Go to Madd Dogg's house in Mulholland and find the pool. The oyster is in the pool.
#12) Next, go to the northeast and find Blueberry. Follow the middle road east out of the town and you should find two bridges, one wooden and one concrete. The oyster is underneath them.
#13) Follow this river east until you get to Fisher's Lagoon. There is a pier jutting from the east beach here. The oyster is at the end of the pier.
#14) Swim following the water to the north to the second bridge just before the freeway overpass. The oyster is under the second bridge.
#15) Head to the Federick Bridge on the right side of Los Santos, which connects to Las Venturas (it's a train bridge). Not including the support beam at the beginning, the oyster is on the north side of the third beam.
BADLANDS & SAN FIERRO OYSTERS
The starting location for The Badlands will be Angel Pine.
#16) From Angel Pine, head east, following the road on the south end of the map, until you cross a bridge. The oyster is underneath that bridge.
#17) From the last one, swim to the north and you should pass underneath a wooden bridge. The oyster is underneath it.
#18) Follow this river underneath the next wooden bridge and it should start to head west. You should pass under a large concrete bridge. The oyster is under it, on the west side of the middle beam.
#19) Keep following this river until it ends and goes out to sea under the bridge that connects San Fierro and the countryside on the westernmost side. The oyster is underneath the middle of the bridge.
#20) Head north of the bridge, on the freeway, until you encounter the first right turn. This should be the road that leads up to Missionary Hill. There is a pool of water right there and the oyster is in the middle.
#21) Next, go all the way to the north to the bridge above Battery Point. The oyster is on the south side of the first set of support beams.
#22) Swim directly east of the last one to find a freight boat. The oyster is underneath the west side of the boat (this boat was in "The Da Nang Thang" mission).
#23) Go all the way to the northeast corner of San Fierro. To the northeast of Esplanada North is a freight boat. The oyster is under the boat, on the west side.
#24) Head south to Garver Bridge. The oyster is between the two support beams out a bit in the waters.
NOTE - The next two oysters are in spots where you automatically get a 5-star wanted level, so it is important that you save before attempting to get them. It is highly advisable that you have full health and armor, as well as a fast boat so you can get away quickly. They are in the naval base.
#25) The first hellish oyster is located under the east side of the aircraft carrier.
#26) Go around the corner and you should see through three areas where one might park a boat. The middle opening has the oyster.
#27) Go to the Easter Basin docks south of the naval base. This is where the crane for the Import/Export is. On the south side of the freighter is the oyster under the ship.
#28) Go to the airport and go all the way to the end of the long runway. It's next to the wall at the end of the runway.
#29) Head to the northwest corner of The Panopticon. Find the beach on this northwest corner and it should be close to it.
#30) Get a boat and head to the inlet just to the east of Easter Bay Airport. Follow it until you get a lake called Flint Water. You should have two options now, a path leading east and one leading south. Go east for now and you should pass under a bridge. The oyster is under the bridge.
#31) Take the other path now heading south. There is an oyster under the first bridge.
#32) Keep going south until you encounter a second bridge. The oyster is under it (this is a train track bridge).
#33) Keep going south until you go under the freeway (double road). The oyster is located in between the two roads, in the middle.
#34) Go south some more and go past the last bridge in the area. Just north of the Los Santos Inlet is an area where the beach meets the cliff-like area. The oyster is there.
DESERT & LAS VENTURAS OYSTERS
#35) Go to the north end of the Gant Bridge on the west side of the map. Find the northermost support structure. The oyster is between that and cliffside.
#36) Go to the northeast and find the boat school. Southeast of that is a small beach area (Skimmer parked there) and there's a rocky outcrop just to the south. The oyster is at the northwestern point of the outcrop.
#37) Follow the coastline to the southeast and stop when you are north of the freighter in the NE San Fierro bay. North and a bit west is a roadside wall that has the oyster in front of it.
#38) Continue following the coastline to the south as you pass under the Garver and Kincaid bridges, then turn left as soon you see the small inlet of water just past the Kincaid Bridge. The oyster is under the easternmost freeway overpass.
#39) Go around the southern tip of Tierra Robada and head a bit north. You should see the ranch used by Toreno. The oyster is in the docking house to the left of the pier.
#40) Head north. Go past the low bridge and also the train bridge, then turn west into the small inlet. The oyster is under the overpass at the end of the inlet.
#41) Get on the road and head towards the Sherman Dam. On the north side are control towers. The oyster is located between the two pillars under the westernmost control towers.
#42) Go northwest of the Sherman Dam to find a small inlet with a wooden bridge. The oyster is underneath this wooden bridge.
#43) Go north until you spot a house with a Skimmer on the right side. The oyster is underneath near the Skimmer.
#44) Feel free to hop in the Skimmer and fly northeast of your airstrip. To the west of the pipe that enters the water is a beach that goes into the water. The oyster is close to that beach.
#45) Now fly all the way to the northeast corner of the map. It's not too far off of the shoreline. If you're having trouble finding it, draw an imaginary 45 degree line from the northeast corner of the map that intersects with the land. The oyster should be somewhere close to the middle of the line.
#46) The next oyster is located in Roca Escalante. The place you need is southwest of the strip mall in Creek and northwest of the offramps on the Julius Thruway East. There is a large building there with a pool in the shape of a guitar. The oyster is underneath the diving board (there's also a horseshoe in the pool).
#47) Go to The Strip section of Las Venturas and find The Visage on the west side. There is a pool in the front with waterfalls. The oyster is located under the second waterfall from the south.
#48) Go south along The Strip until you find Pirates in Men's Pants. On the right side of the moat in front of the entrance is a skull shaped rock. The oyster is in front of the rock.
#49) Continue south along The Strip until you find Come-A-Lot. Find the pedestrian walkway in the front and the oyster will be on the left side of it.
#50) A bit northeast of the pyramid in Camel's Toe is a place called Pilgrim. There is a pool out front and the oyster is located in it.