If you find yourself covering a lot of local water polo, you'll start to notice certain referees that are used a lot in the area games. If there's one in particular that you notice, or who you know also coaches a team (some high school coaches also may be refs/umpires, often for the college-, and even junior-national and national-team levels), I'd try to talk to them/get to know them a little bit, and get a contact phone number, so you can use him as a resource when needed.
In an esoteric sport like water polo, that is more technical than it seems, and where there are a lot of little-known rules and unseen, underwater action, it can be really helpful to have a knowledgeable voice to talk to in order to add details and depth, particularly with regard to technical aspects or stories related to club- or elite-level games/athletes.
I had one referee/coach who was always a tremendous, knowledgeable, articulate source for me when it came to water polo. He was somebody who worked high-level, even Olympic, games, as well as being a coach of a successful high school team and serving as a referee on the local scene. So, although I didn't turn to him real often for rules/elite insider type stuff, I could, and I did, when needed, and it always added great nuance to the stories.
People connected to the so-called "minor" sports, even the refs, are often very open, knowledgeable and willing to share. I always loved covering them, and water polo actually got to become a particular favorite.
