Besides fish, you can catch all sorts of things whle fishing across Azeroth. Some of the items are containers- for example a Mithril Bound Trunk. These containers will contain other items. One item in particular is the Weather-Beaten Journal. This item teaches the ability Find Fish. This ability enables tracking of nodes (or pools/schools) on your minimap as well as causing them to display the "gatherable item" effect - that is, the sparkles you see around an item that can be looted or gathered.
Your best bet in fishing up containers is to fish floating wreckage. Any wreckage has a chance to cause containers to drop, and any container may contain the journal.
Poly, the Turtle
A must have on my mage, you can fish up Tome of Polymorph: Turtle. Any class can fish it up, but only mages can use it. That's a shame, since I would love to have polymorph on my hunter.
In the Terrokar forest, in lakes reachable only by a flying mount you will find nodes called Highland Mixed School. In these you have a chance to catch the elusive Mr. Pinchy. Mr. Pinchy grants wishes if you speak to him. One such wish is the coveted (and achievement-worthy) Magical Crawdad Box.
As with much of the unique content in WoW, this item is somewhat easier to acquire than it once was. Still, it's great fun to reflect on the blood, sweat, and beer invested in chasing these when they were fresh new challenges.
The Sea Turtle Mount
Nothing beats the awesomeness of an underwater mount. With patience, you can fish up the Sea Turtle mount, and impress everyone. This mount is the objective of the achievement: Turtles All the Way Down. There are all sorts of guides to catching this mount, but really it's a matter of luck and patience. Nuff' said - go get one.