As in any team sport, in Underwater rugby you need a mix of individual skills ranging from ball handling to an eye to read the game.
The main skill that gets you started is obviously swimming. In fast paced games the breathing technique and ability to stay below the surface are critical for success as the game is more dynamic while the ball is played under water rather than on the surface.
The challenge in reading the game comes from the 3d nature of the game. Your opponent as well as your team mate can be anywhere in the 3D space and to follow this development of game reading and spacial awareness are beneficial. Reading the game correctly is important as it is hard to perform long and accurate passes with the ball in the water. If you are able to position yourself in the proper way this usually also conserves energy for you and your team mates as the game can be played by passing the ball rather than fighting on the surface.
Skills that help and are “easy” to enhance are your swimming speed, your upper body strength and your underwater stamina.