A regular swimsuit is a bad idea in cold water

Don’t.

You lose heat about 25x faster in water than in air. A standard swimsuit offers zero thermal protection. You will shiver uncontrollably within minutes, your muscles will seize up, and you risk hypothermia long before you finish your swim.

What you actually want is a neoprene wetsuit. Thickness depends on water temp: 3mm for cool (60°F/15°C), 5mm for cold (50°F/10°C). If you’re going below that, add a neoprene hood, booties, and gloves. Some races ban wetsuits above certain temps, but in genuinely cold water, wetsuits are the standard.

If you’re doing a short polar plunge or a 100m sprint and just want to say you did it, go ahead in a regular suit. But