School me on guilds....

[Einhander]

Guilds with number usually contain nothing but basics that don't know how to properly man a ship. They sit on the wrong cannon constantly, never follow attack directions, etc.
A good guild (like the ones in the guild sub-forums) helps rid you of that.
Personally I know to not sit on one gun unless the others are full (rarely happens) and run from one gun to another depending on the target in relation to the ship.
Just last night I was on a fully-crewed galleon and boosted my cannon skill from level 5 to level 9 in under an hour. Excellent cap'n (shout out to KK!) and I think everyone was over notoriety level 15.
If you're low on sailing skill having a good crew can boost that up in levels extremely quickly. Bad crew and too high a target and you're sunk quickly as well.
A guild can also help out on some of the higher level quests when you have to take out something 5+ levels about you, you hollar for help and someone equal or higher to the critter's levels can help you take 'em out.