Here is the full list of all 14 Luna's Fishing Garden achievements. It takes around 2-3 hours to unlock all of the achievements on Windows.
Find Poppy in a cave
Clean up the archipelago
Use balloons 3 times
Catch every fish
Catch an oarfish
Build 10 bird nests
Help Gustav rebuild a bridge
Feed Sam avocados
Complete any of Nigel's field journals
Plant 15 lotuses
Catch every fish in one spot
Fail to catch 10 fish
Catch the same fish 3 times in a row
Jump off the island into the ocean 3 times