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