This is my first time with this, but:
Ability: Sap Sipper or Huge Power
Sap Sipper causes a nasty surprise to all grass types who hit it, making it immune to taking grass type attacks altogether, while increasing the attack stat.
Huge Power just basically increases the power of all it's physical attack moves.
EV spread: 252 HP, 252 Attack, 4 Speed
Item: Shell Bell/ Leftovers/ Choice Band
Aqua Tail/ Aqua Jet/ Waterfall
pros and cons
Aqua tail hits hardest of these, but as lowest pp, and has some chance to miss. Aqua jet has least power, but adds priority, most pp. Waterfall has next to most powerful hits, has chance to flinch, and has good pp for the power.
A powerful stab that might lower the physical attack of it's target.
Bounce/ Ice Punch
Pros and Cons
Bounce hits them naughty fighting type that may not be hit well against the fairy type move, Play Rough, such as Croagunk, and Toxicroak, and use poison type attacks, which are super effective to our pretty little Azumarill, and has a chance to paralyze, but it has only 5 pp, and 85% accuracy. Not to mention, moves like Gust, Twister, and similar moves will hit harder than normal, while up in the air. Ice Punch has a chance to cause freezing, but isn't as powerful as Bounce, and leaves risk against fighting types that know poison type attacks.
To cause added protection against poison types, and protect it against Electric types, while reducing speed. But it has weaker power, but without the risk of getting hit by massive ground type attack, while using Dig. Dig has more power, can avoid most attacks, but if hit by moves like Earthquake, Bulldoze, and similar attacks, will take more damage than normal.