Sam eyes the collar for a moment before looking Castiel in the eyes and doesn't try to take it off. The water will probably damage it, but it can't make the damn thing any worse. The collar's cheaply made and not well cared for in the first place.

He takes a few moments to get undressed himself, dropping his clothes to the side. There's nothing sexual about his nudity, either. He's done this with Dean, after all, when one of them is too injured to wash alone. Then he reaches out, taking Castiel's hand and drawing him into the shower and under the spray.

He'll look over the wounds soon, get to that plug and deal with it. For the moment, though, he just lets Castiel enjoy the not-quite-scalding hot shower.