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