Of course, it's all different, in a fantasy world...
First, look at the size and type of the ship... Small ships have small crews. Ships without catapults/trebuchets and/or ballistae won't need "gunners" or Artillerists, etc.
A more minimalist crew might look like this (with some (N)PCs filling more than one role, simultaneously):
Captain (General command of the whole ship, but delegates everything).
Lieutenant(s) (The Cap'n's chief delegates).
Boatswain (Usually spelled Bo's'n, as it's pronounced - in charge of working or sailing the ship - If the Cap'n orders full speed, he orders the sails raised, turned so-and-so to catch the wind, when to tack, and all the other details).
Master/Navigator - Also probably has Professions (Sailor & Cartographer), as well as Knowledge (Geography). As such, either a Ranger or Wizard is probably most likely, although Clerics or Bards might also work. He tells the Helmsmen what course to set.
Quartermaster (Controls how larger ships are steered; delegates to Helmsmen).
Helmsmen (man the wheel, in shifts).
Cook/Chef(s) (Hobbit(s)?).
Healer/Physic/Leech/Doctor/Surgeon(s) (Clerics of water religions, probably).
Translator/Diplomat (Undoubtedly a Bard, in 3.x).
Purser (Responsible for food, clothing, supplies, & cargo aboard).
"Carpenter" (In 3.x, he is actually a Profession (Shipwright), as well. Carpentry might be used to fix a broken mast, taffrail, etc.).
Coxswain(s) (Profession (Boater), as well as (Sailor). Somebody has to have skill with the smaller craft used for getting on and off ship, especially since many Sailors couldn't swim!).
Sailmaker(s) (Turns canvas into sails, and keeps the sails in shape. Profession (Tailor)).
Blacksmith (Repairs metal fittings, and makes whatever breaks).
Shipwright (See "Carpenter". Prevents the boat from sinking, fixes broken masts, etc.).
Sailors (Deckhands man the deck, Topsmen man the more dangerous rigging (including your lookouts), and would have more skill in Climb and Rope Use, although all Sailors will have the latter, in addition to Profession (Sailor). Note that, while it may seem odd, many Sailors could not swim, so don't feel that they HAVE to have the skill! Also, Deckhands need not have Climb).
Marines ("Tell it to the Marines! The Sailors will never believe you!" Landlubbers, mostly, who are transported via, and sometimes fight upon, ships. Mostly warriors, with a few Fighters).
Artillerists (Man the ship's weapons, if any).
Master at/of Arms (Trains the Sailors in weapons use, and passes out arms, keeping track of who has them).
Weaponsmith (Keeps the ship's arms in good shape).
Caulker (Has primary responsibility for keeping the ship from leaking).
Swimmer (May also be the caulker... This poor soul gets to go overboard and attempt to make repairs to the ship while it is not only in the water, but also underway! He will have high Swim skill, as well as Profession (Shipwright). When there is no swimmer, some poor sailor gets basically keel-hauled to do the task!).
Cooper (Yes, a barrel-maker (and fixer)! All water, and most food on board, are stored in them!).
Cargomaster (While the Purser keeps track of it, the Cargomaster is responsible for storing it, keeping it from shifting, keeping things from getting smashed or strowed, preventing it from unbalancing the ship, ad nauseum. On smaller ships, he may also be the Purser).
Ratcatcher (They're a plague aboard ship (sometimes literally), and a youngster (and sometimes a ship's cat) was usually employed to do nothing else but hunt and trap them... In 3.x, a Druid would be an interesting choice).
Naturalist (Again, in 3.x, a Druid/Ranger would make a nice choice).
Most of these positions had men working under them... Hence, the Surgeon had a Surgeon's Mate, a "Loblolly Boy" (or nurse); the cook had several others under him, the Lieutenant(s) had First Mates, who had Second Mates, etc.
Women aboard ship were considered bad luck.
"The Devil" was the longest seam on the ship, running from prow to aft. It often had to be cleaned of barnacles, and then chalked, so that it wouldn't slow the ship's progress. While the "Swimmer" could do this job, it was traditionally left to the ship's biggest goof-off, who, if he couldn't swim, was keel-hauled... A rope was tied to him, and he was given a scraper and the chalk, then thrown off the prow and allowed to work his way aft until the float attached to the end of the rope reappeared, and was fished out of the water, and he was hauled back aboard. Since it often took many trips to chalk (or "pay") this seam (known as "The Devil"), many Sailors drowned. Hence the expression "There'll be the Devil to pay!" was a dire warning.
As for the spellslingers, they would pro'ly be classified as "Artillerists", or maybe "Marines". Note the Concentration checks for getting off spells in a storm, aboard ships in rough seas, etc.
