In reply to batterj2:
Sounds similar to canoe polo.
Green card - Minor infraction, going for the ball with a paddle when it is in hands reach of someone else, obstruction etc. You get the card and a telling off but carry on.
Yellow card - Getting a couple of green cards, you have been warned or something that could have been more serious such as ramming the side of someone with your boat where someone could get hurt easily. Sent off until the end of the half.
Red card - Getting a couple of yellow cards or doing something dangerous, ramming someone's side at speed, pinning someone under water, Sent off for the rest of the match (possibly tournament depending on how bad it is).
Before getting the red card, unless you do something very dangerous you get a whole load of warnings and often will get removed by your team.