i think your problem is that your wearing too loose of a speedo. I'm a collegiate water polo player and swimmer, and I wear extremely tight small speedos from the Spanish based company Turbo. And its nearly impossible for me to obtain errection. So cut out the middle man, and just kill the boner, nobody needs more drag!
I agree with waterpoloplaya...I still compete in swimming and the suits are VERY tight! ouch! in fact I can't wait to get the dang thing off! It's the same for jamers...they will show. My training suit is the only one that give me room to grow if I need to...but i'm so tired after practice it's the last thing i'm thinking about...lol so it's not happened.