We can easily assume that in this day and age all high level athletes perform some sort of strength training in their program. Just look at any professional team and you will see that they have strength& conditioning( S& C) coaches as part of their faculty while Collegiate level sport in the USA also will routinely have a team of S& C coaches.
Athletes used to be able to succeed at a high level merely based on their genetics and natural talent. This is no longer the suit as athletes are getting bigger, stronger, faster and more powerful each year. Appear at any sport and you will see the evolution that has occurred. For instance, Tennis is now dominated by powerful servers as opposed to the serve and volley players of the past; ice hockey was formerly dominated by the likes of Gretzky but now the super stars of today are much bigger and stronger and this translates into a much more physical game; even Water Polo is changing.
The key plays in a Water Polo match are all about velocity, strength and power. Just think for a moment about the strength and power requirements to drive, catch and shoot the ball or the wrestling match that goes on at the centre forward position. The ability to win in these situations has very little to do with how fast you can swimming 400 m or how many 100 m you can swimming on a 1:20 but on your strength and power. Getting stronger will absolutely help you win more of these battles.