do we wear something under board shorts? do we have to wear underwear?

Answer 1

You wear what you feel comfortable wearing.

Some guys wear nothing under board shorts, some wear a speedo type bathing suit, some wear underwear.

One comment on wearing underwear under board shorts - assuming it is cotton underwear (doesn' t matter what style) keep in mind that if you go into the water, while the board shorts will dry while you are in the sun, anything under those board shorts (especially cotton) will take longer to dry. That could make for some uncomfortable times.

One other note: if you are somewhat "endowed" you may want to consider wearing something under those shorts to help keep things in place.