Immune to Polymorph effects. Increases swim speed by 50% and allows underwater breathing.
They call him Flipper, Flipper...
Also known as a Sea lion, druids obtain this form from any trainer at level 16. (There used to be a quest line to obtain it, while it can still be completed, it is not mandatory.) One can only use the Aquatic Form while swimming, but can travel normal running speed in water and will not run out of breath (water breathing). Take note that this 50% increase is of the 66.6% base swim speed, hence giving a 33.3% bonus. Thus, the net effect of this form is to allow you to move at 100% normal running speed in water.
The downside of this form is that it has no defensive capabilities whatsoever, meaning that in a combat situation it's only really useful for escape. Outside of combat however, it has a large number of potential uses. The speed bonus means that it can serve as a fast form of travel if a given destination can be reached via water, and the greater presence of rivers in Alliance territory means it also gives Horde druids venturing into that area with numerous opportunities for concealment, reconnaissance, and rapid escape as mentioned above. The form can also be exceptionally useful for druids with the Herbalism tradeskill, as it allows more rapid harvesting of the herb Stranglekelp, which is valuable on many servers.
To gain the Aquatic Form skill you once had to travel to Thunder Bluff or Darnassus where you can begin a series of quests starting with the quest A Lesson to Learn. You then continue your journey by travelling to Moonglade to get further in the quest chain. Once you had completed the quests you needed to speak to the DruidTrainer that initially sent you on the quest in order to learn the Aquatic Form spell.
While in Aquatic Form, you cannot cast spells or talk to NPCs. You are also considered a Beast so spells that target Beasts can be used on you (Hibernate for instance).
Your caster form will start regaining mana 5 seconds after you change to Aquatic Form.
As with the other travel based forms, the druid cannot drink, eat, or use items.
Use this whenever you go into the water. It is a useful technique for escaping PvP opponents, as few can equal the Sea lion's swim speed and few can hold their breath long enough to catch you. However, without Glyph of Aquatic Form you are still slower than a hunter's pet underwater.
Be slightly wary of Warlocks underwater. They can breathe with a buff and their skills aren't restricted like the Aquatic form Druid's are.