I don’t like the idea of competing with NPCs for rewards (especially if it’s really difficult). All these “rush ahead of everyone in 2 minutes” mechanics don’t fit the life sim genre at all, it turns into a race 
But underwater hunting and different treasure locations — yes, I’m on board with that. The main thing is just that they don’t spawn randomly where they might be inaccessible for harvesting — there’s always that risk. However, if we go the same route as with sand piles, which appear in set locations but you never know if they’ll be empty or if you’ll find something (in the underwater version — chests or distinctive seaweed bushes on the bottom) — then yes, that could work 
I think this might be something similar to Предпочтительное ношение купальника на территории пляжа , isn’t it?