There is no mention of this in Wikipedia's Water Polo page.
There's plenty of unsupported stuff on the Internet about what a dirty sport water polo is (ironic really, considering it's played in water ... ) and making claims about players sharpening their nails, etc., but there are also plenty of ostensibly authoritative pieces saying that these rumors are greatly exaggerated. On this page in Quora.com, one player writes, "any polo player worth their salt would never consider [sharpening their toenails] ... In my experience, dirty players quickly develop a reputation that actually works against them. It's easy to take cheap shots under the water, and dirty players end up getting more than their fair share. I believe that this is why there is somewhat [he means something] of a code of honor in polo ... "
On the same page, a player and referee writes, "Some leagues [my italics] require a nail-check by the referees before a game ... It is the player's responsibility to make sure their nails are short while playing, whether or not a nail check is conducted by the referee."
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