I've been shouting this sometimes while fishing and actually think this could be a fun idea!
With a fishing dungeon we can be fishing out in sea on a large fishing boat. We have certain tools and baits to bring in large quantity of fish.
Boss battles would be stopping a giant squid/Shark/Tuna/Anchovy from destroying fish by weakeningwith hook launcher, and capturing it.

Disciple of Land/Hand raid could be a large party of Fishers fishing food, culinarians cooking, lumberers gathering wood for tables, etc.

Little sleepy to go into full in-depth details I had but I'll just go with this.