Resource and upgrade system
Fishing merge games commonly connect board play to resources such as coins, bait, catch value, aquarium collection, or gear parts. In the official description, discovered marine creatures can be caught and placed into an aquarium, so puzzle progress and collection progress reinforce each other.
The best upgrade is usually the one that improves your most frequent action. If most progress comes from steady merges, upgrades that support spawn quality, storage flexibility, or catch value tend to compound well.