Grand Theft Auto: San Andreas/Oysters
You will probably find your first oyster during The Da Nang Thang. There are fifty to collect after that.
Collecting all 50 oysters results in increased lung capacity and sex appeal. That's right, the reward for all that swimming is that you are able to swim longer.
The increased Sex Appeal allows you to start dating any girlfriend regardless of her preferences for muscle/fat, and that she will invite you for Coffee right after first date.