Direct answer
The official gameplay description says fish can be sold for money, which is then used on equipment, weapons and other items. Use money to solve the next progression problem.
A practical spending rule
Before buying, name the exact quest, boss, island or efficiency problem the item solves. If you cannot name one, keep the money until the route is clearer.
Bonuses and gambling content
The Steam description mentions doubled bonuses from trick gun spins. Its mature-content disclosure also mentions gambling with fish or in-game currency and states that no real money is used. Exact values and mechanics remain version-sensitive.
Fact check: August 24, 2026 · Primary factual source: official Steam app data (App 4001890).