Aquatic Form is a druid form learned at level 18. Also known as a sea lion, it can only be used 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 its only use is 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 numerous opportunities for concealment, reconnaissance, and rapid escape. 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.
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.
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 is usual when swimming, the druid cannot drink or eat in this form.
When used in conjunction with Sea Legs, Aquatic Form travels at 450% of normal swim speed. Glyph of Aquatic Form does not stack with this effect.
Prior to the Cataclysm, players gained Aquatic Form once they completed a series of quests starting with the quest A Lesson to Learn. Once completed, they needed to speak to the trainer that initially sent them on the quest in order to learn the spell. Completing this series of quests is no longer mandatory in order to acquire the ability, as it is learned automatically now.