Candiru - the fish that will jump/swim into your peehole and suck your blood - seriously!
The more you know! God sure has proven a unique sense of humour creating this little fellow. The story is wikipedia- and straightdope-verified and comes with pictures of the removal operation. In case reason alone just isn't enough to convince your kids not to piss into the goddamn pool:
"The candirú or canero (Vandellia cirrhosa) is a freshwater fish in the group commonly called the catfishes. It is found in the Amazon River and has a reputation among the natives as the most feared fish in its waters, even over the piranha. [..]
It is feared by the natives because it is attracted to urine or blood, and if the bather is nude it will swim into an orifice (the anus, vagina or even—in the case of the smaller specimens—the penis and deeper through the urethra). It then erects its spine and begins to feed on the blood and body tissue just as it would from the gills of a fish. The candiru is then almost impossible to remove except through an operation, usually involving the amputation of the area.
"On 28 October 1997, one of us (Samad) attended a 23-year-old man from the town of Itacoatiara on the Amazon River who sought medical attention with obstruction of the urethra, having been attacked by a candirú. [..] The fish penetrated the victim's urethra while he was standing in the river urinating, actually emerging from the water and entering his penis, filling the entire anterior urethra. He reported trying to grab hold of the fish, but it was very slippery, and it forced its way inside with alarming speed. The candirú's forward progress was blocked by the sphincter separating the penile urethra from the bulbar urethra. With the passage blocked, the fish had made a lateral turn and bitten through the tissue into the corpus spongiosum, creating an opening into the scrotum."
pictures of the removal surgery