How to swim underwater in GTA San Andreas for iOS

It is based on oysters collected and repeated diving.

Whenever the player enters water, a white bar denoting the player's oxygen appears on the game's HUD, beneath the Health and Armor bars. When underwater or in a sinking vehicle, the bar will gradually empty; when exhausted, the player's health will be reduced instead, leading to eventual drowning. The oxygen bar is replenished by returning to the surface of the water. The player's lung capacity determines how long the player can stay underwater, and is improved both by continuously diving and by collecting Oysters. The player can tread water and swim on the surface of the water indefinitely.

To unlock "Wang Cars" -the car dealership in Doherty- & the associated missions from Caesar, you must first complete all driving school tests, and the 'Yay Ka-Boom-Boom' mission (possible spoilers, click at your own risk).

Jethro will call, and tell you the place has gone on sale. After purchasing it, you will receive missions from Caesar to 'stock up' the showroom.

Source: GTA Wiki