Grand Theft Auto: San Andreas/Missions/Oysters

You will probably find your first oyster during the Da Nang Thing mission. There are fifty to collect after that.

Locations
Spoilers
 * At the stern of the Da Nang boat
 * At the stern of the Export boat
 * In the pool of the Tiki Motel, Pilgrim, Las Venturas
 * In the Glen Park pool, Los Santos

Reward: $100,000