I've been waiting to finally have something to post here that is genuine and worthwhile, hope you all enjoy it!
Here is how to fish up a Giant Sewer Rat at the same time as doing your Coins achievement:
what you will want to do is stand as pictured in these 2 images
when you are up against the wall it will look like this:
as you see in the first image sometimes casts will land outside of the fountain and look like you are fishing in the cement. what I found was that regardless of where the cast lands (either edge of fountain or cement) it will count as an eligible place to get coins and the Giant Sewer Rat.
I stumbled upon this by complete accident. being a gnome I had to stand here while fishing in the Dalaran Fountain due the to "water is out of line of sight" error from being too short for the normal fishing spot. imagine my surprise when I was a few coins from coin master and a giant sewer rat popped up in my loot window
Also, I was using a fishing bot to fish here as I'm sure most people that do the coin achievement do. I recommend Mr. Fish It very highly from the Bots and Programs section if you are interested in one also.