A creative swimming challenge

Have received a very creative challenge. It requires me to go to a non nude beach (there aren't many nude beaches in NZ because nudity is i*****l).
I have to go to where there are a reasonable number of people swimming and use water soluble body paint to paint on a pair of board shorts. I then have to walk into the water and when the paint has washed off. Make my way as best as I can back to my clothing.
Can anyone expand on this