I have shot a bunch of it as my wife plays. Standard action stuff applies. Fast shutter, light might be sketchy. I would suggest a CP to block the glare off the water, it will even out your metering and will make the water contrast better with the players. Of course, that will cost you some light, stupid tradeoffs! Your 400mm will be fine. You'll also have alot of opportunity to catch bench action (coaches, refs and players) as they are all right on the pool deck in front of you.
If you haven't watched much water polo, its a mixture of basketball and soccer (football), except that much of the action is taking place under the water, and only the hardcore fans and players will be able to follow much of that action. All of the whistles are a total mystery. Same rules apply for following and capturing action. Most of the best stuff will be around the holeset (the center), right in front of the net.
Here's one from the archives, waaaay back in 2005.
Camera Canon EOS Digital Rebel XT
Exposure 0.003 sec (1/320)
Focal Length 300 mm
ISO Speed 400
Exposure Bias 0/2 EV8-27-05 047
, on Flickr