I have never watched a complete match of water polo or know many of the basic rules, but I've watched a lot of soccer, football, baseball, basketball, and watched my kids swim at the rec center a lot, so I think I'm more than qualified to offer some analysis.
1. Players need to create more confusion. Have lines popping in and out of the water at practiced cadences, but arrhythmically to create confusion for the other team. It's kind of like foosball spinning lines, but instead it's random. The opposing team won't know where all the defenders are. Being in the water, this could be scary.
2. Underwater screaming. It's unsettling.
3. Splashing. Get water going all the time. The equivalent of when you were a kid and your dad went on his back and kicked up a ton of splashes at you. If 4-5 players did this the offensive player wouldn't know what to do, and would be easily dispossessed of the ball.
4. On offense, a flying V formation, but with the guys in front in the V splashing with their legs like in step 3 as they get closer to their goal. I haven't tried it out in the water yet to see if it is a viable way of swimming (splashing forward and at the same time moving that direction with your arms) but these guys are Olympians. If that doesn't work they might have to skip leg day more often so that their upper body strength can power them past their kick-splashes.