Welcome to Naples! We've been here about 5 months and really enjoy it. i played club WP at the Academy, and would love to play here, but haven't seen anything. There are 4 US/NATO pools here; the Support Site pool is all 5 ft deep except the small dive pool, Capo pool is same, not sure about the NATO pool but from a distance looked same, as well as Carney Park. There is a really strong swim team here that travels all over Europe and has a great time, so even if no WP, that could suffice!
This link:
http://www.cnposillipo.org/ looks to be to the local Naples team with different under X age teams. Right side of page you'll see "nuoto & pallonuoto" in the pink button (waterpolo). Use Google translate or Bablefish to translate page.
Good luck! Todd