I wear a 30–32 inch waist in most men’s pants/shorts and that translates to a medium. I don’t wear them at my “true waist” and you probably won’t either I’m guessing?
From a quick look it sounds like size 16 is probably a little smaller in the actual waist and a little bigger in the hips, so a medium should be pretty close for you as well.
I know you’re probably aware, but there’s more the difference in cut than just the length. Men’s shorts tend to be more straight from the waist and don’t have extra room for all the lovely curves of a lady’s middle.
I’d grab a medium, but you can always grab a large and cinch them up as well if you don’t mind wearing them loose. Board shorts are supposed to have a little room to them.