Much like in land rugby, different positions require different skills from the players. In general, size and speed is the key. Speed is obvious, but what's up with size? Reaching a certain level you'll need it if you want to be able to tackle opponents or pull the goalie off the basket. Don't forget, the ball itself is heavier than an average medicine ball!
Due to playing breathhold, flawless swimming technique and a nice lung capacity is also important if you'd like to be one of the best.
Furthermore, compared to american football for instance, more endurance is required as there are no brakes during the matches. Also, as teamplay is very important and there is no communication in the water, the players have to be able to make individual decisions which correlate with the team's tactics.
So how do the best underwater rugby players look compared to average humans? Much like football or rugby players, just a bit healthier and water-ready.
This a picture of a Colombian team called Koral. Colombians tend to play a faster, more agile style.
This is a picture of Molde from Norway. One of the best teams nowdays, and their defender, Iver Bjornerem was voted best player of the year in 2014. 186 cm tall and weighs 125 kg.
But don't worry if you are not one of the ideal types. The great thing about UWR is that the chances of injuries are low so you might as well practice with the bests as long as you don't mind loosing. Also, a well organised team with disciplined teamplay will always outmatch a group of strong individuals who can't cooperate.