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