A combination of the word swimming, and incest.
When people on your swim team date each other, or date ones siblings. Different variations include, dating multiple people on the team at different times or dating someone on the team that a different team member has alread dated.
Emily-Omg did you hear the news at practice today?
Andrew- No! What happened?
Emily- Well Erin and Dylan broke up and now Dylan is dating Mandy while Caroline (Erins sister) is dating Dylans best friend Taylan.