When you do missions for Wu Zi in San Fierro, you will have to do a mission than involves swimming. if you have done a lot of swimming before this mission you will proceed with the mission and do it. BUT, if you do no swimming barely at all when it comes time to do that mission a video will play with CJ and Wu Zi playing video games and CJ admits that he can't swim. Then you'll have to go swim a lot in order to come back and do the mission. (I got to give a shout-out to my brother Toupoc)