Sounds like you are asking more about light. My comments are going to be more about composition.
Waterpolo is more of a "headshot", so even though you feel close, you have a small target. Even when I'm close, I shoot with my 400 most of the time. Since you want the eyes in focus and their arms will come up, I set my AF point down a little. This gives the arms room to come up.
Next, for most sports, I like to get really low. For water polo, this can give you a messy background, so I sometimes go up a little. Not high, but just high enough for a clean background.
I also agree with the comment above, get closer and try to shoot tighter. My personal guide is the ability to count eyelashes, so close.
Light, well, the hard thing here is that gear matters and a 2.8 lens will help a ton. A 2.8 lens will cut your ISO in half much of the time. I start with shutter speed. I love 1/1000th as a base line. Then, open as wide as possible and set ISO for the best exposure. I'm ok with some things being blown out as long as the face is exposed right. If you need to boost in post, you will boost a lot of noise. The more you get it right, the better your noise will be. What else can you do? Lower the shutter. Will you get motion blur? Sometimes yes. When they are treading water just holding the ball, you might get lucky down to 1/640 or 1/500th. Shooting... you'll want closer 1/2000th for the strong players, so time your shorts.
If you can get exposure right, also try shooting jpeg with in camera noise reduction. Increase the sharpness and test it out. I shoot a lot of stuff over 12,800 and 10,000 is common for me. My body (Nikon D5) handles it well, but the jpegs come up very nice. Better than I can easily do in post on RAW images.
Flash... depends on local rules. The strobes we use in pro arenas are not cheap. High power and very short duration. It is the short duration that freezes the action. Setting them up in arenas can be tricky and you will want multiple units. Some arenas have a dozen or more. Many people instantly think flash is bad for the athletes, but I've asked a long and none ever know unless it is an on camera pop-up style. Today, most cameras have good enough high ISO that strobes aren't worth it.
So long post, but I think you can drop shutter and time pictures with less motion. Get closer and work on composition, but do light and composition at different times until you get comfortable with each. Build your confidence and your images will get better every time.