This happened to me when I was 16. I went to the beach with my 3 friends on the fourth of july, and there were tons of hot chicks, much of whom went to my school.
The day before the drawstring came out of my bathing suit, but I thought nothing of it. I waded out quite far into the water, and out of nowhere a huge wave hit me! I literally did about 4 summersaults under the water, before I was able to get up. I was just happy I survived, it was so shocking and the water was freezing cold. Then I realized that I felt wierd, and that my bathing suit had come off! I was still pretty far out so "that part of me" (which isnt terribly big to begin with, and was severely shriveled by the water) was still not visible. I frantically searched for it for about 20 min, becoming very scared. Just then I realized I had drifted down pretty far and I didnt even know where I was. I had no choice. I waded closer to shore as far as I could while that part of me was still covered. Then I had to do it - I was going to sprint to the bathroom. It looked like I had a clear path and I am an extremely fast runner. I took off like a speeding bullet when - bam! I smashed right into the girl I had a crush on, butt naked, and knocked her and myself over. I got an erection and became extraordinarily red. I was stunned, I just sat there for what seemed like hours. "Wow thats tiny!" she exclaimed. Everyone began to laugh. I regained control and sprinted the rest of the way to the bathroom. There I found some paper towels to cover myself with. I had to still run though to my towel, wherever it was. So I supposed I should just get it over with. I ran out of the bathroom, with a crowd of girls hollering and making embarrassing comments and slapping my butt.
Finally I found my towel after about 5 whole minutes of running and torture. I gathered my clothes, dropped my towel (again accidentaly exposing myself) - followed by screeches,and ran like hell to my car, whimpering. I was teased throughout school for months and still am.
This is by far the most embarrassing moment of my life.