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