Ocean Chub
When the water hits your crotch and turns your flaccid into a full-blown boner. Silky trunks make it even more annoying.
I got the ocean chub so bad I had to stop swimming and just float there like a moron.
The chub started when I was diving and my trunks got pulled up.
My chub was so strong I had to hold on to a buoy to keep from getting pulled under.