First off, no, i do not have the salty achievement. Secondly, yes, i have completed 'The One that Got Away'... although before it counted (i caught the rockhide strongfish).
Now, on to my suggestions:
First off, Mr. Pinchy's magical crawdad box HAS to be changed. Either make it so each Mr. Pinchy guarantee's a crawdad box, remove the achievement from the list, or have it shared with I Smell a Giant Rat (so that catching either pet unlocks the achievement). I'm on my 4th Pinchy ATM (fingers crossed!) and i am yet to get the box, it's frustrating enough that he's a 1 in 500 catch, but then you only get a 1 in 5 chance of getting him
Secondly, remove the 'one that got away' achievement from the salty title. Before you all rage at me, let me explain that this is one of the most frustrating achievements in game, as it is completely, 100% reliant on sheer luck. While the crawdad box and even the sewer rat have skill involved (either pure fishing skill, or the fact that you have to keep moving around to different pools) this one is just a one in million catch, that someone who has all the other achievements wont get after hours and hours of fishing, but some pleb who just got his first fishing pole will get it on his first cast.
HOWEVER - this achievement is so big, and has such a small chance of completion, i think it warrants it's own achievement 'reward'. Here are a few suggestions:
'Nat's Extra Thick Fishin' Line'
-Fer Really Big Fishes - Enchants your pole with a further +10 fishing skill
'Impossibly large Pile of Fish Fillets'
-It can't possibly be infinite, can it? - Think Everlasting Under spore pod, it summons a stack of food - 12hr cooldown
Other ideas include some kind of fishing chair, and some kind of boat 'mount' that you can drive around water and fish from