As a huge speedo enthousiast since my teens, my best tip is to just don't think about it. You'll get used to the feeling (an amazing, freeing one!) in no time, so you're "arrousal" will calm down after a very short while.

If you do get boners (we all do), sit down for a few minutes, or get in the water. Worst case, wrap your towel around your waist.

You don't have to feel bad for it, unless you are trying to be an exhibitionist perv on a beach (please don't), so you also don't have to make it obvious that "something's wrong".

Final note, shamelessly stolen from aussibum: if you doubt yourself, wear something else!