He looks over, studying him, because Cas has to be talking about when Dean couldn't find him because Cas ran out on him instead of staying and letting Dean help.

Used to be Dean enforced his personal space bubble around Cas, but that's gone out the window lately, the more Dean's needed reassurance that Cas was there, and the more he let himself need that. When he feels Cas brush up next to him though, the first thing that pops into his mind maybe isn't the nicest. But it sounds like the best idea.

He nods, turning again to look back out at the water.

"Unless you can't swim," he says, and that's all the warning he'll give before he nudges Cas hard, trying to push him into the lake.