The Fishing Rewards could certainly use shaking up, it's true... Here's a few suggestions to add to the Sea Serpent trophy;
A Void Parrot:
Every sailor loves parrots, so why not have a Void Parrot to go along with the Void Crabs and Lobsters. When mounted on a perch, it will eat boots and shoes (in it's void stomach) thus saving fishermen much annoyance... and potentially selling more Britannian Ships! A high end reward of course, or perhaps you could add Void Parrot bait at the same chance as the high end Legendary Bait, and you have to go unto the jungle and sprinkle it on a biscuit to have a chance to summon and catch said Parrot...
48 Pounder Cannon:
The artwork for this is in game already, but it's not been used at all.
Pieces Of Eight:
Coins that when dropped on
any parrot cause them to squawk "Pieces of eight!" randomly for 5-10 minutes. A minor prank to play on friends. Medium level difficulty so as not to get too annoying. The coins stack of course, take note devs!
Boots full of fish:
A special container that holds
only fish, but have pack beetle sized weight limits; allows fishers to carry fish to and from their boats whilst using vollems or other traditional defenders from sea serpents... High level reward
Charbydis/Osireadon Heads for Trophies:
When a 120 fisher casts the net or bait for these, they get a chance at the head too which can be mounted and the date, caster, and whether it dropped any artifacts (to anyone, not just the fisher) listed on it
Huge piles of cannon firing material:
'Nuff said.
A Plank fer yer mates to walk off.
Ship deco, possibly only for Britannian ship depending on code, but shows mounted on the side of the ship, and anyone walking off it triggers a drop and splash animation (and they get teleported back onto your ship, or possibly your own home (it's a house teleporter in disguise)
I'm sure i'll think of more whilst bored and at sea... stay tuna'd!