Used to swim with a guy, just before the pool closed at the apartments I worked...
He laughed about this, "never true, big hoax..".

So, while he was taking a quick shower, before jumping into the pool, I placed 3 Purple dye tablets in his little coin pocket. 

With the swimming pool in the middle of the courtyard, overlooked by dozens of apartments, pool lights on at night...  that stuff just kinda "glows" fluorescent, and lingers for a minute or two.