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