I make it a point never to travel without packing a swimsuit. As I have noted before, I could travel to the interior of Antarctica in the middle of the austral winter, and I'd still pack a swimsuit. Swimsuits don't take up too much space, and one never knows when one will be confronted with a body of water in which one can swim.
The last time I was in Switzerland, Sweetums and I jumped off the Wasserwerkstrasserücke into the lovely Limmat on a warm May day. The water in the river was still very cold, cold enough to "burn". I don't anticipate any impromptu dives off a bridge in December, but there is always the opportunity to check out some hot springs.
I don't often offer unsolicited advice, but seriously, folks, if you're going to travel, you must pack a swimsuit. Chances are, you'll use it.