Here is the complete guide on how to become a Water Ninja:
If you are a member of clubpenguin and have your ninja mask, head over to the ninja hideout and buy the amulet. Then you can enter the card jitsu water lair. Walk up to sensei’s chair, and he will give you instructions on how to play and a starter deck. He will tell you your journey. Your journey is to defeat other penguins and earn the legendary water ninja outfit and gem. The outfit consists of the wave sandals, the waterfall coat, the torrent mask, and the helmet of oceans.
After you successfully earn your water ninja suit, challenge the sensei and try to earn your legendary water gem. Sensei is fast, but the first few times only, after about 4 rounds, he gets slower. Here is what your card jitsu water suit should look like once it it completed.
Congratulations you have just successfully learn how to become a water ninja!
(Credit to ECCP http://elitecheaterscp.wordpress.com/)