During the Spearfishing Mini Game, you will use what are known as Spearguns to shoot and kill Fish. You will automatically unlock one speargun at the beginning of said Mini Game, but you will have to unlock the five by completing specific Substories. Each speargun has three different Stats (Spear Count, Strength and Aiming Speed), with the best spearguns having a higher number in said stats. In order to successfully complete all Fishing Substories, you will need to obtain the final spearguns, the Kiwami Speargun and Ahab's Revenge.
How To Unlock/Obtain All Spearguns
Standard Speargun: Obtained at the beginning of Spearfishing Mini Game
High Impact Speargun: Obtained after completing Fishing Substory #47
High-Capacity Speargun: Obtained after completing Fishing Substory #48
Kiwami Speargun: Obtained from Mama after completing Substories #41-46
Modded Speargun: Obtained after completing all Fishing Substories
Ahab's Revenge: Obtained from Black Market Merchant