GTA 5 Mission Walkthrough: Daddy's Little Girl
Michael's kids don't like him that much, and that makes him sad. Michael takes it upon himself to take his son Jimmy outside and go on a bike ride together.
After the bike race to the pier, Jimmy reveals that Tracey, his sister, isn't the angel that Michael thought she was, and that she's hanging out with porno producers out on a boat, that's not that far from the pier.
After swimming over to the boat and "rescuing" Tracey from the party, you'll have to outrun the angry producers on a jet ski.