>>96225014 (OP)
Having done it, here are my findings.
* Don't use D&D. The combat assumptions are all wrong, the magic is all wrong, it will take too much work to convert.
* Don't replicate D&D in system B either.
* To feel piratical the apex combat builds must be swashbucklers with pistols. If you allow axe chads, snipers or the like too much power your parties will not have recognisable pirates in them.
* You must balance action scenes and acrobatics with combat: this keeps players from over encumbered builds or using heavy armour.
* Pistols and rifles add a whole new dimension to combat. Everyone has a likely one-hit attack in their pocket, and when they choose to use it is a massive tactical element.
* A ship has a large crew. Players with their own ship can throw junior ratings at problems. Design for this, and allow it. It's not one locked door, it's a series which must be broken down by teams of men, etc. Individual action can be provoked where the PCs do not want to be spotted (big groups are obvious) or where superstition holds back all non-PC crew.
* Shift your fictional base away from Tolkien pastiche (elves, dwarves) and towards later pulp and Indiana Jones. Magic must be rare, or just this side of plausible ("luck" and the like), and sourced from powerful entities with their own motives. Supernatural horrors in the third act only.
* Life at sea was gross and brutal in our history. Pirate media downplays it a little, but must have an element of "most people want to settle down or sail only to accomplish a worthy goal"
* Non-human player races are probably better served as being variant humans (born at sea, in a giant swamp, in the choking fumes of a city) with exaggerated traits. Avoid real world races not for sensitivity but because the arguments WILL stall your game.
* Finery and jewelry are required for access to society. Full battle rattle is not acceptable in towns unless you sack them.
* Magic should be mostly a costly tradeoff. Often Faustian.
