Water Mastery. One of the 8 available Masteries you can choose from.
Water Elemental Mastery allows the ninja to learn Water techniques. Water is a very balanced nature and as such can be useful in many situations.
Water is overall a really nicely balance mastery. It has many advantages to it while not really having any drawbacks to it. It has instant cast techniques, good damage, piercing, AoE, a stun and a substitution as well. These things make it one of the best masteries to choose to start with.
For PvE this is a fantastic grinding mastery. Since it gets piercings pretty early one it makes grinding on mobs pretty easy. It also gets AoE to hit any mobs that move off too the side.
For PvP it's really top tier, since you have instant cast techniques it can make hitting targets easier. Coupled with a stun it makes this mastery deadly. What really completes the circle is also having access to an extra substitution. While it's good in 1v1 it's also really good in group fights as well since the piercings can hit multiple targets.