While finding all 50 Oysters in GTA: San Andreas will require some work, the benefits are more than worth it - in addition to it being a requirement for 100% completion.
You'll net $100,000 in total, boost your lung capacity when under water, and will be able to date any potential girfriend in the game - regardless of looks or sex appeal. You'll also be able to re-date girls who have dumped you.
They are are often found underwater or near to water, such as on beaches, in canals, under bridges, or even in swimming pools.
Enlisting vehicles such as speedboats or the amphibious Sea Sparrow or Skimmer will make the task a lot easier.
Here are the locations of all the oysters in the game.