Water polo is a sport which requires many different muscle actions over
all planes of motion to be successful.
Traditional training tends to neglect exercises which occur in the transverse
plane. The transverse plane bisects the body into upper and lower halves.
Your exercise program should include many rotational movements moving your
body through different, changing ranges of motion. You can see oblique exercise videos to find a variety of exercises which help
strengthen your muscles in the transverse plane.
Since water polo is a full body sport, exercises which work multiple muscle
groups and actions would be a great way to increase your strength, and
especially endurance. Instead of performing exercises which isolate a specific
muscle group you should use combination exercises which you can view at combination exercises videos.
As far as supplements go, they are exactly what the word says, a supplement.
As far as improving your performance, it is better to use supplements to ensure
you get all your nutrients which you need.
A good multivitamin is a must. The more times a day you must take it, the
better because your body will get a constant supply of the antioxidants and
B-vitamins which are needed for intense training and recovery.
As with any sport, the best way to improve in water polo is to practice.
Playing year round with a solid weight training program and balanced, healthy
diet rich in nutrients will make you the best you can be.