I don't think I have met any local surfers who wear anything under their 7mm wetsuits. We all went commando. When using my 4/3 for windsurfing, kiteboarding, and wakeboarding I have never worn anything under. Although if you want to look cool and not have comments like "Twigs and berries" you need to wear a pair of board shorts OVER the wetsuit.
For diving when I first started in a 7mm wetsuit I wore nothing under (Then quickly changed to a drysuit). For warm water diving I am typically found wearing boardshorts and go commando under my shorts. I stuff the shorts into my 4/3 mm wetsuit if boat diving. If shore diving and I had a car available I would probably go commando in my wetsuit.
The trick is knowing how to roll the top edge of the towel down to make sure it isn't going to fly off. If the towel did fly off, then I would just continue on as if nothing happened. I have nothing to be ashamed of, and we all have naked bodies underneath.
I wear fleece pants for a base layer in my drysuit and always change out of my street clothes into them under a towel. Two layers is enough in the groin area, and the cath offers all the support I need
Why is it our culture is so ashamed of something that is so natural. Maybe it is my Finnish up bringing, but I was taught there is nothing shameful about a persons body, no matter the shape or form. Wear what you want or nothing at all, personally I sure don't care. If someone gets naked in front of me I don't say a word.
....in case you're in an accident and are sent to the hospital.
The EMT's cut my partners beautiful French lingerie off, when she got hit by a car riding her bike. Anyhow at the hospital they put you in a gown with no back. It is a little breezy moving down the hallways.