"Oh, uh-- no problem. I mean, it's... y'know, I'm glad you want to learn this stuff." If he keeps carrying on the conversation, he's got something to think about that isn't Dimitri rubbing sunscreen over his chest. He should... see if there's anything he hasn't unpacked yet that he should. Maybe he missed something in the bag, and can look there instead.

...It's only moments later, however, that he looks at Dimitri almost warily, because he possesses enough pattern recognition to see where this is going, but there's also... really no way around it. "Uh... yeah, that'd be best. If you're swimming your back will probably be exposed to the sun a lot."