As far as I know magery is good to backup fishing with. My questions are,
as a fisher what skills do I need?
Fishing is the only mandatory skill you need for the profession, however being able to kill sea serpents and other monsters at sea without needing a bodyguard is recommended. Archery or Magery is best, but you can manage comfortably with swords/macing/fencing (just watch your health).
how many skills can I GM?
700 skill point cap. You can GM up to 7 skills.
how much is a boat?
Small boats are about 10.3k, medium boats about 12k, and large about 13.5k. Small is all you need (and it's easier to manage with regards to docking and moving around obstacles).
where do I get one?
Either at a provisioners shop, or at an anchormans shop. Anchorman shops are usually near a towns docks.
best place to raise fishing at?
Shore from 0-70, deep-sea fishing afterwords (fishing anywhere 12 tiles away from shore). You CAN GM fishing from shore and thereby avert danger (and macro unattended legally), but it'll take longer. Hiding or Invisibility is recommended while fishing to avoid PK's, thieves, and possible monster pulls.
At 90 fishing at up, from deep sea, you will have the chance of pulling up a Treasure Map (level 1 only from what I've seen) or a fishing net. On the side, both make for a nice flow of income. At GM (and only at GM) fishing you will be able to pull up Message in a Bottle, or MiB. Pop those open and use a sextant to sail towards the location of the coordinates listed. Once you're there, fish over the side of the boat to pull up various shipwreck items like bone, barrels, paintings, etc. Eventually you'll pull up the treasure chest.
Level 1 chests: Disappointing. 1k gold, few gems, few spell scrolls few magic items (weak ones too), few reagents.
Level 2 chests: Nice. 2k gold, nice abundance of everything above (with luck, the wands might give you some serious profit).
Level 3 chests: Loaded with 3k gold and a huge abundance of gems, scrolls, reagents, and magical items (high-end modifiers being possible here). Hope for that +25 Silver/Vanq Hally and not a shepherds crook.
Have fun.