It helps to really enjoy fishing, because that damn Sea Turtle seems really rare and hard to fish up out of a Northrend fishing pool. Fish very regularly just for fun. A bit since the turtle was added in the recent patch. So far this week caught about 1000 fish just doing some regular fishing for tank food buffs and re-stocking up on Fish Feasts for raids. Rest of fish to sell of course. I usually stay fishing long enough to catch 100 minimum of whatever I'm there to catch or more.
So lucky me I finally fished the Sea Turtle from a fishing pool fishing for Dragonfin Angelfish and got the achievement as well. Mayby not as elusive a trying to catch the most elusive Mr Pinchy, but just as hard the effort it does takes to catch the turtle unless the RNG really likes you for some reason. I can now say I've caught them both. The Sea Turtle really does swim fast as well. Yeah Baby!