Hello!
I think it would be nice to have fish box functionality expanded based on its nature by making it able to produce more fish of same kind if 1) 12/15 slots (80%) are filled with same kind of fish 2) there are no other fishes in the box. If these conditions are met, new fish will spawn every 2 days and take up one of empty slots (which means amount of fish farmed this way would be capped at 15-12=3 slots). Considering that you put medium fishes for max hunger output, a single fish box would provide you only 25 hunger per 2 days, or 1/6 of your hunger lost, which is still pretty balanced in my opinion (you'd need 6 fish boxes to sustain a single player on a boat and the space on the boat is very limited).
This feature seems to provide extra incentive to go ocean fishing - for your effort, patience and preparation to catch initial amount of fishes and not eat them all - you are eventually rewarded with a compact and portable food farm that doesn't depend on luck (lureplants) or being first to raid the world for mob drops (pigs, bunnymen) and can be used by all characters (monsters like Webber/Wortox and vegans like Wurt).