Neoprene swimsuits are the only real option.

Yes, but you’re not looking for a swimsuit. You’re looking for a neoprene swimsuit — which is basically a wetsuit cut like a swimsuit. Regular swimsuits, even the ones with “thermal” linings, don’t insulate in water below 70°F. They just get cold and clingy.

The insulation comes from neoprene trapping a thin layer of water, which your body warms up. That