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