Jade Owl

This is an EXTREMELY good pet despite first appearances of being weak, being the best pet in the game to counter Aquatic pets. As a Magic pet, it takes reduced damage from Aquatic abilities and most of its attacks are Flying (and fairly good flying attacks at that), which are strong against Aquatic. As of this time, it is the ONLY magic pet with such attributes, making it a very unique pet indeed. Note, however, that as a crafted item, the only quality you can find it in is uncommon, so be prepared to have to upgrade it with a Flawless Battle-Stone.

This is also a fairly good pet for leveling weaker pets. You can find very large numbers of Aquatic pulls near the lakes in Vale of Eternal Blossoms, and these pulls will have either 2 of 3 or 3 of 3 pets being Aquatic, with the final pet sometimes being Flying. This pet can easily smash those pulls with little effort, allowing you to bring two low level pets that don't contribute but soak up experience.

[item=90625] from the last [faction=1269] daily of the day, or
[object=214388] if you happen to loot an [item=87779], may contain
[item=90470] which teaches
Jade Owl which creates
[item=82774] which teaches
Jade Owl, this companion spell, which summons
[npc=61877], the battle pet.

Edit: The recipe is now also available from [skill=755]'s [item=95469], added in patch 5.2.

I cannot think of a better pet against aquatics, since it is strong to them, and they are weak to it. I highly recommend purchasing one and when you level it, remember that since it is magic, it cannot be damaged more than 35% in one attack, so go ahead, take on some high levels! This one isn't fragile! :)