Nothing! If you want to be more comfortable then its time to ditch the boarshorts or the speedos, and go swimming in nothing but rubber.
For some wearing extra clothes under a wetsuit is a necessity. For triathetes being ready to get on the bike and have a fast T1 transition mean swimming in just the wetsuit isn’t a luxury they can afford. But if your an open water swimmer, or a triathlete just going for a training swim then there really is no excuse.
Some may wear speedos or a swimsuit underneath for ease getting changed, but if your treat yourself to a changing robe then this really isn’t necessary and you”ll be a lot warmer getting changed.
Its difficult to think of a reason why to wear anything under an open water swimming wetsuit. If you’ve got a super tight and well sealed wetsuit then there is a chance when you jump in the wetsuit may shrink wrap between the nether regions (especially for men!) letting in a little bijt of water will resolve this, but wearing speedos wont make this any better.
Finally rash vests aren’t designed to be worn under a wetsuit, better to just wear the wetsuit with a good anti chafe or wetsuit lube.
Disagree? let me know in the comments!