[I'm assuming on rereading that Darkwind is a boat.]
I think it'd be cool if he could use units to power up ships. I thought of two ways to do this, just depending on the functionality/what you would need to add.
A) He begins with str 5ish, buildable with the optices probably, and starts with boarding party and "Pirate Recruiter," an ability like donal Lugh that recharges--in his case when ever he captures a ship. But Recruit Pirate works in any city with a harbor, and gives 3 units: A sloop (caravel with +1move & +1 cargo space), a pirate boarder (basically boarding party with lower strength, and perhaps engraged) and one random other unit such as: a pirate gunner (only if blasting powder is known) which can be sacrificed to give any ship in the same tile an "expert gunner" promotion for +15% str; a pirate alchemist (only if alchemy is known) can be sacrificed to give any ship in the same tile an "firepots" promotion for collateral damage; a pirate carpenter (only if construction is known) that can give a ship "march" promotion, maybe renamed; a pirate doctor (only if medicine is known) that can give a ship medic 1 promotion; A first mate (only if celerty is known) that can give a shoip blitz; and maybe others like a pirate ghost giving fear, or a pirate whaler giving capture animal (for the krakens). Other than the "join ship" ability on these guys, they would be like the pirate boarders without the boarding party promotion. (Maybe give the pirates a free combat 1 promo if the city recruit is used in has a tavern

)
This method would be fun I think, and simulate a legendary pirate putting in at the first port after a rough battle to grab a crew from whatever is availible. Downside is that it would require a handful of seldom seen units and might so be a waste of resources.
or
B) Rather than the recruit ability, give him a "press gang" ability. This would be somewhat similar to create flesh golem. Use the ability in any coastal city, must select an owned ship and friendly non-animal unit. Creates a "fully crewed" version of that ship that has (most of) the promotions of the press-ganged land-lubber, but otherwise the same stats as the ship. The fully-crewed promotion would prevent the ability from being used twice on one boat. (The land unit would disappear)
Some promotions would not be able to be transfed: Anti-unit type (like shock)& terrain defense for obvious reasons. Mobility, because ships have navigation, and it wouldn't translate to water.
Chaneling would not, but Sorcery/Summoning and magic sphere pormotins
would, so if an adept was added to an arcane barge (which starts with channeling 2) it'd get additional spells, but a privateer would not. City raider or garrison would not, of course. Undead or demon could, meaning a ghost ship that could learn fear.

Hero probably shouldn't, but if you dragged a hero onto a ship, his other promotions would make for a formidable vessel. Enchantments ok, maybe? Oh, and summoned would too, if you tried to 'cheat' and add a str 5 imp to a boat, it'd take the boat with it in a turn
This would be a fun ability without adding the pirate units, but it may be a pain to code? Dunno