Since you are starting out at level 78 you have much more flexibility in your fishing options. I would suggest trying to kill two (or more) birds with one stone to make your fishing leveling less boring.
If you want to garner a lot of fishing achievements, you could start in either Ironforge or Orgrimmar (whether you are Horde or Alliance) and continue until you get
Old Ironjaw or
Old Crafty, respectively. Additionally, you can try for the achievement of
catching a rare fish.
If you want to try and obtain a
Sea Turtle mount, you can fish in schools around Northrend hoping to get lucky.
Convenient location-wise is to fish in the Dalaran Fountain to work on
The Coin Master.
You can learn what fish make up the food that buffs your class the best and just work on building up stacks of those.
Or you can just find a location you like and sit and fish there in comfort.
Just remember, being near the trainers make your leveling faster as you can learn your new skills quickly. However, since they are located in cities, you won't be catching any unique fish that you can use in leveling cooking along with fishing.
Edited, Oct 31st 2009 1:34pm by CountFenris