[Rin grabs the t-shirt, pressing it against his face. he seems to be letting the conversation lull into silence, but after a few moments in which Dave's shirt is thoroughly ruined (rude??) Rin pulls the cloth away enough to say]

I almost quit swimming. Like, mostly twice, but also a bunch of times.

[it's not something he wants to confess to, particularly, but Karkat's a friend. a close one, if Rin is being honest with himself -- maybe the best friend he's made so far. he didn't think less of Rin for being unable to fight, or code, or whatever infinitely more useful skills he's lacking here; he won't think less of Rin for this, either]

I think the people who have to struggle to master something that's a part of them earn that mastery more. It just makes you better, that you had issues with it. It means you thought about it, decided you loved it, worked hard, and overcame things.

[red eyes lock on Karkat over a shirt that's becoming similarly flecked red]

Mastery isn't about perfect execution, every time. It's about effort and passion.