The type of fish you turn into is a frenzy, the Warcraft version of a piranha.
Has a 30-second cooldown after equipping before it can be used. After this, it has no cooldown.
Consumes no mana to transform, since the item does the transformation and works for all classes.
(unlike druid seal form, which can be temporarily unavailable to a druid who is too low on mana.)
When you shift into fish form, you regain your breath as if you were above water. It'll typically take a few seconds to regain your breath.
While not as fast as a druids aquatic form, a druid must drop aquatic form to drink a [Swim Speed Potion], whereas you can use the such a potion without breaking the "Fish Form". Travel speed (percentage of normal running speed) with a Swim Speed Potion is 156%. However, a druid using [Glyph of Aquatic Form], is still faster.
If you are a Rogue it should be noted that you can [Stealth] underwater first, then use the trinket which does not break your [Stealth] (useful for going afk underwater).
Like the druid's Aquatic Form, if you are transformed into a fish and jump out of the water at the surface, even if you immediately dive back into the water, your buff will automatically dispel itself.
Does not stack with the larger bonus from [Sea Legs].
Patch 3.0.8(2009-01-20): The former 5-second cooldown was removed and the increase in swim speed was increased from 25% to 35%.