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
Collect 150 coins.
Buy Imperator fish
Buy Goldy fish
Collect 75 coins.
Reach 300 points.
Reach 200 points.
Get into the mouth of a shark.
Collect 25 coins.
Fall on a flying bird.
Obtain a coin and die a second later.
Reach 100 points.