From time to time the DM might need to determine if a NPC professional succeeds at a difficult technical skill or craft - does the blacksmith make a suitable masterwork weapon on the first try, can the sage answer an obscure question, that kind of stuff. As I collate notes for my future (Caribbean-based) saltbox setting, this extends to solving for skills like sailing, navigation, and gunnery.
I've seen the simple 2d6 reaction chart from classic D&D turned into a skill resolution chart. It assumes a modifier scale of -3 to +3 using ability scores. (Historical note: Charisma was capped at +2 in the Moldvay rules, but this was changed for Mentzer and the Rules Cyclopedia). For professional skills, I'm using the following descriptors to determine appropriate "ability modifiers":
Unskilled characters may not even get a roll in some cases. The basic reaction chart goes like this:
Skill Resolution (roll 2d6):
2 serious failure
6-8 partial success
12 great success
An example usage: if the players hire a skilled navigator instead of a master navigator for their ship, they'll pay less in salary or shares, but there's a higher chance of getting lost on the way to that hidden island. As I develop the nautical rules, I'll work on specific guidelines when hired experts would make checks and how the dice results translate into specific outcomes.
For basic D&D, I like decoupling professional skills from the class and level system and treating them as background skills for NPCs; it keeps the major emphasis for class advancement on adventuring and developing adventurer capabilities. Nonetheless, I can see the players asking if they can develop any degree of professional skill (especially these nautical skills) if they spend enough time on a boat.
This post from an older blog (Skills: The Middle Road) provided some recommendations for education around professional skills, so I'll use those values as a starting point and will see how it goes:
Learning a Profession:
Skilled - 1 month, 1000gp
Expert - 3 months, 3000gp
Master - 6 months, 10,000gp
If a player character commits to spending time in the rigging, learning how to furl and unfurl the sails, close-haul a ship, tie the right knots, swab the deck, and so on, they can be a skilled sailor after a month of effort and a bit of money on the side. (For sailing, assume the money goes to gambling, carousing, repairs, clothes, basic gear, dues, and so on; ordinary folks pick up the same skills at a slower pace without all the money spent).
Alternatively, I can look at a hybrid game like ACKS, that bolts a new-school inspired feat and proficiency system onto the old school chassis. Navigation and Seamanship are already represented as choices in ACKS, and Gunnery is easy to add. ACKS also addresses when someone would be qualified as a "master" by stacking extra picks on a proficiency. I'm not sure the high-fantasy tone of ACKS is what I'm going for, so the actual decision on rules flavor is for sometime down the road.