Strangely there were quite a lot suggestions on Crew gaining experience etc and no such proposal here.
Simple, but quite effective system for implementing Crew experience in game (expanding basic idea by @Sea Archer)
Currently Crew is expendable and available in any numbers in any ports (even enemy ones). Only situation when Crew matters is in Battle instance (where if you are undercrewed you'll have problems) and sometimes in OW when you have no spare Crew.
Meanwhile IRL it was the Crew quality that made difference between victory and defeat and determined the whole course of naval war - even with plenty of good ships you couldn't do most things without enough experinced seamen. The most know examples are wars (1739-1815) between Britain (with average ships but also large pool of able seamen) and France and Spain (better ships, but a way less able seamen to crew them).
So the main limiting factor to fleet size (besides pure resources and money) was a amount of trained and experienced men needed to crew the ships - hence impressing sailors at sea from traders and foreign ships etc...
Principles and Mechanics
Each player have Rank XP (from battles and sailing) and Crafting XP (from building ships) - let add to it also his Crew XP. That's the main idea.
You gain some XP from each action you have (even if you lose it). Gained XP counts as XP gained also by you Crew (Crew XP). This XP assigned to your Crewmen.
Only those Crewmen will get those XP that you have in the end of fight before leaving (if you get sunk or blew up as a Fireship and lose all your Crew, you won't get any Crew XP).
Received XP added to your Crew XP and the amont added is defined by proportion between Crew survived the action and total Crew numbers before it (see example).
Crew XP will have levels (just like Rank and Crafting) - currently I suggest 5 of them: Recruits, Landsmen, Seamen (no bonuses - standard crew), Able Seamen and Jack Tars.
Low levels will get you negative modifiers (inexperienced Crew), higher levels - positive ones (veterans). Difference between lowest and highest levels should not exceed 10-12%. Also you will always have 40 crew (basic free crew) as standard base level (no bonuses).
All crew you recruit in port are inexperienced men (they have no XP) and their including in your Veteran Crew will lower overall amount of Crew XP, probably also lowering Crew level (see example).
There should be no difference between ships (just like with Knowledge slots) and anti-altfarming mechanic also shouls apply to this.
Your are a Flag Captain, having 500 crew and your Crew level is 3500. You sailed out of port in your Trincomalee having 325 crew and engaged enemy AI Frigate. You won the battle gaining 250 XP and in the end you have only 280 crew.
Then new total Crew XP is defined in following way: 3500 (basic XP) + (250 (gained XP) multiplied by (280 (surviving Crew) divided by 500 (basic total Crew))) = 3500 + 250*(280/500) = 3640.
Next you returned to port and decide to replenish crew, recruiting the lost 45 men (with 0 XP). After that your ne total Crew XP defined again in following way : 3640 (initial XP) multiplied by (455 (initial crew) divided by 500 (crew after recruitment)) = 3640*(455/500) = 3312 XP (so even after victorious action you can lose a bit)
More sense in using Rum for resurrecting Crew even in the end of Battle. Also splinter protection will have more purpose
Surrender will also have much more sense now, as you are saving your veterans from certain death.
If you lose hard fought battle (against superior numbers for example) you will gain some benefits from you skill and courage.
Gaining XP when you are at highest rank will have sense.
Less incentive in fireshipping, especially with 1st rates, cause you will lose too much crew.
Probably less incentive for boarding actions with potentially heavy losses.
Screening (if it stays) will probably changed from simply dying in DLC and capped ships and buying time to smth more sensible, cause you will need to save your crew for other day.
Sailors Lifes Matters!
Main problem with introducing crew with experience and better stats - it can (and will) widen the gap between successful and usually victorious gankers players and the rest, especially noobs getting farmed at capitals etc. Just as in reality of 1792-1815 with experienced and veteran British crews against French and Spanish, not having enough sea training because of blockade (from which Brits gained their naval XP), therefore constantly losing.