Can father and son swim nude together?

To be blunt, and on the face of it, all other things being healthy and ordinary, there isn't much of a problem here. This is assuming that the arrangement is very temporary and infrequent. This might turn out to be convenient while traveling, for example. There are certainly cultures outside of western ones where nakedness is simply nothing to get twisted about. But obviously if there are any ulterior motives, or if either person is the LEAST BIT uncomfortable, then put on your briefs, boxers or pj's. If there has been any history at all of difficulties around nakedness, sexuality or sexual boundaries, then they have no business whatsoever ever sleeping together whatever the circumstances.

Ulterior motives could come to light very easily: One of them has to put effort into persuading the other that this is OK; one of them becomes uncomfortable and the other becomes miffed; the nakedness is part of a progression of activities that leads to touching; the bed is simply too small but is the only accommodation possible for them both and still one insists on sleeping naked.

As I think and write about this, in general it is probably not a good idea, even if there may be circumstances where it is innocent.