I just wear swim trunks when I think of it; Underwear when I don't. The
only reason for anything really is modesty.
I use cotton a fair bit and it does not matter. The reason cotton gets
a bad rap is because as an outer, or vented inner coating when it gets
wet it holds the water and allows it to evatorate. Evaporation is the
principle used to keep your air conditioner and fridge cold. That
eveporation and close contact with the skin will kill you in a cold
climate. Under a wet suit there is no where for the cotton to release
its water so the evaporation ( drying ) does not happen , that means
you are fine.
In warm climates Killer Cotton is the best thing in the world. Here in
eastern Canada, in the Labrador current it is not something I would
wear on the ocean ( ever ) . In Nova Sotias Bras' Dor Lakes I likely
would seek out a cotton T shirt.
I, by the way; normally wear a dry suit.
The important thing is to be comfortable and safe. If you are getting a
wet suit and asking these questions you will likely be fine.
I would try to take a course or join a club in your area.The
information is good and often free.
All the best
Alex McGruer