Well, at a beach, yes. Easiest way to move around in the water, and deal with the heat.

[ And technically they're not knickers but he won't go too much into detail. He does glance down at himself and wonders if Jacob thinks Jim's actually in some form of underwear right now. ]

I am a fairly good swimmer, actually, just not against giant sea bass. Also, I'm Captain of a starship, not a sea ship.