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