I was reading your ideas and love the brit turn in idea.. Love It.. heh
Heres some of my ideas.
PAINT for our boats.. to color them.
How about boat building..
You buy a basic boat.. and a boat tool kit.. the materials to improve your boat are actually fished up.. so for a vast improvement you'll need to fish alot.. i mean ALOT. Improvements could be battle sails, or new heads for the front, PAINT (hehe), anchors, decorative chests (like white chests with a red stripe or lid or such.), oars, ship shields, cannons, steering wheel thingy,wood, nails, windows,port holes, ect. heh. But make the changes exclusive to fishing and no other skill. Adding items also prepares fishing for later adding bods to the skill.
A sea dungeon.. a dungeon that you must first pass thru a sea cavern to access. Naturally there needs to be harbormasters or someway to prevent people from blocking...but its an idea.. Accessable only by boat.
Bait... for fishing. How about worms. You dig them up and they act as a consumable to increase your odds of fishing up sea serpants. (a consumable)
Remnants of a lost city.. a flooded ruined city lost in the ocean containing a sm. dungeon containing a gardeners paradise of beautiful gardens and lush tropical plants, some unique monsters and a quest person or persons. They give special seeds or plants to travelers from the new worlds. An ancient underwater subteranean race.
How about a portal a vast waterfall that you pass thru that teleports you to the ishtar lake for fishing.
Additional pirate caves that allow you to fish inside as you wander... Each level giving greater danger. (similar to shame dungeon) maybe in tunuko islands.
Fishing up a special type of armor 6 pieces similar to how your able to find armor pieces in dungeons.
Removal of small fish or allowing them to be used for alchemy or cooking or for use in aquariums. I think most of us eat them just to rid ourselves of them or toss on corpses.
Fishing in the deep sea might allow fishing up of an elusive black gem.. used by imbuers or even add a weight to it for tinkers to turn into something like a globe with your name and stones attached. (we like those for player contests, like big fish)
Fish up statues, rocks, drifting shrouds,