There are four races spread around the various waterways of Los Santos. All take place on a Seashark jet-ski and you must come in the top three to complete the race. All four races are available from the start unlike other races that need to be unlocked first. They all count towards the 100% completion total. Also note that for some of the races, you must be using a Seashark to be able to start them. A few will allow you to enter without one by climbing onto the empty Seashark.
The races themselves are not too difficult. The hardest part is learning how to maneuver the Seashark. The handling is quite unique so it may take a bit of time to get to grips with. The key thing to note is not to turn the steering stick too tightly or you will end up spinning around too far. Even in sharp corners, slow your pace a little and use the control stick lightly until the back end has caught up. From this, make any adjustments you need. You can also make a very sharp turn in the water using either RB or R1 but I would only use this sparingly.