Here is the full list of all 13 Nimble Fish achievements. It takes around 0-1 hour to unlock all of the achievements on Windows.
Buy whale
Buy all fishes in the game store
Buy Imperator fish
Collect 150 coins.
Collect 75 coins.
Collect 25 coins.
Buy Goldy fish
Reach 300 points.
Reach 200 points.
Get into the mouth of a shark.
Fall on a flying bird.
Reach 100 points.
Obtain a coin and die a second later.