Hiding is useless and a waste of skill points, without stealth.
I have butchered my t-hunting template into a fisherman. I had GM hiding (you can thump it just before the chest pops), and I left it on the template. Hiding has been a real convenience for going after those rare fish in hard to reach places like twisted weald, prism of light and the Labyrinth.
For the most part you don't want to kill anything in these places or it will just respawn on top of you. It has been easier to lead the mobs off. so I can pre-cast invis and lead things around. Problem is, most things in those places reveal you as soon as you invis.
When they reveal you though, they don't aggro again straight off. With protection, combined with my so-so suit, I tend get caught into a lot of invis/get revealed/invis/get revealed loops. However with hiding I can thump the macro to hide as soon as I get revealed. So I'll sit there until the hiding timer is up and plan where im going to lead mobs on the next step. Once I have a fishin hole cleared out ill run for it and thump the hide macro again.
As for sea serpents and deep sea serpents, I have a frostwood fishing pole made with a yew dovetail runic. Frostwood gives the spell channeling mod and the runic craft yielded a roll with fast cast cancelling the penalty plus a little SDI and a STR boost. So I keep fishing right through battling any unwanted deep water guests. I'm currently hoarding all my nets and mibs. I don't use this char for pirate hunting. My pirate hunter is just a straight up ABC archer... honor, EoO.. yada yada yada.
The fisherman template is:
115 magery/eval/fishin
100 hiding
75 med
100 cart/inscription
so... it could definitely be better if it were a pure fisherman but it works very well as it is.
Just keep in mind, if you are going to earn your fishing scrolls the hard way, you are going to spend ALOT of time fishing for monger quests and gains after the scrolls start popping in your bag. The only real threat you have during this are serpents and any beasties that wander in your dungeon fishing holes. you wont be fighting constantly and none of the mobs are really that challenging. What matters most (to me) is to keep casting my line and being able to hold a lot of fish. Having to switch out a pole for a bow and sacrifice weight for ammo is inconvenient.