Here is the full list of all 25 A Fishy RPG achievements.
Legendary Fish #1
Legendary Fish #2
Legendary Fish #3
Legendary Fish #4
Legendary Fish #5
Legendary Fish #6
Legendary Fish #7
Legendary Fish #8
Legendary Fish #9
Legendary Fish #10
Legendary Fish #11
Legendary Fish #12
Legendary Fish #13
Legendary Fish #14
Legendary Fish #15
Legendary Fish #16
Legendary Fish #17
Legendary Fish #18
Helped the surface fishermen
True Ending
Talked with a helpful pig
Exploded a shark on the surface
Caught 100 fish in a single run
Caught 200 fish in a single run
Caught 300 fish in a single run