Long sleeves, long pants, and no underwear are all you can wear under a swimwear.
So long as you have a swim cap, you can cover your swimsuit.
But there’s a catch.
The cap must be a swimcap with a slit at the bottom for your underwear to be visible.
Also, the cap must have a waistband, which means that if your swim cap is too big, you’ll end up wearing it over your underwear.
Read more about the swimwear industry: