Hi everyone.
I'm creating this thread in an attempt to help ye on your quest to finally get your greedy hands on a Sea Turtle!
You will find a lot of speculation and theories about the best way to catch this mount in various sites. Here's the best way to get your achievement!
1. Fly to Howling Fjord and head to Winter's Breath Lake, north of Skorn. Some of you may remember this place from the Argent Tournament dailies.
2. I've been told you can always find fish in pools regardless of skill; however, just in case, i went there with 600 skill and a lure.
3. From now on: fish in the pools of Fangtooth Herring contained in said lake!
-Why should you farm in this place?
The pools of Fangtooth will have a chance to yield the rare fish Dark Herring, needed for the achievement [One That Didn't Get Away]. Both the turtle and the fish have a very small chance to appear, but fishing for two rare drops instead of just one could make it more bearable. Furthermore, even if you already have a rare fish like the Herring and don't need any more, this place is still a very good choice to fish in: in my experience, there's no competition there.
I sometimes found the odd fisherman trying to spot a Herring, but they all left within 20 minutes, and the pools respawn fast.
-How can you be sure it's the best spot?
There isn't a place better than another one to catch your Sea Turtle. However, this place is empty; you don't risk finding enemies or other fishermen most of the times.
There's a theory going around the forums, that is, you have better chances fishing in Pandaria through a http://www.wowhead.com/item=85973/an...-fishing-charm (Ancient Pandaren Fishing Charm). Do NOT pay attention to these theories.
I caught my Sea Turtle in Winter's Breath as the THIRD catch in a single loot panel, along with a Herring and a Suckerfish. I didn't have a charm (not like it would've worked in Northrend). Random is random.
CONCLUDING REMARKS
Have patience... a lot of patience. I farmed for a week before finding it. All you have to do is... don't burn yourself out with constant farming, don't pay attention to theories and supposed ways to increase your chances, and do something else while you wait for the Fishing Bobber to splash.
Good luck!