I am tinkering with something similar right now, with the elf-equivalents being virtually illegal outside the borders of their own empire.
So, here’s what I would suggest you consider:
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First, discuss it with your players and make sure you’re not going to piss any of them off by doing this. If any of them were planning on playing said race, make sure they’re okay with the impact on their play style.
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Consider the storytelling conflicts you want to explore with this. What encounters do you want to put your players through, and why? What themes are you looking to explore?
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Consider the larger impact on other parts of your world. Try to make this more than a point that exists in isolation and a vacuum.
Here’s my thoughts from when I ran GoSM:
Ghosts isn’t really a campaign. It’s a really un-connected series of individual mini-dungeon crawls. In particular, if you were hoping for a focus to be on the three factions within Saltmarsh itself, I will warn you that almost none of the modules have anything to do with that. The leadership divisions in Saltmarsh are just kind of there to help DMs build on their own homebrew stuff. Despite much being made of the Sea Princes, they’re more or less unmentioned in the rest of the campaign, as are the other two factions.
Notably, the modules’ antagonists break down into two major categories:
In my case, I decided I liked undead as a final antagonist better than an Aboleth. The entire thing became a plot by Orcus - my ultimate BBEG - to drown and slaughter everything in the Saltmarsh region. Everyone else - Syrgaul Tammeraut, the Aboleth S’gothgah, the Sahuagin - were either intentionally or unintentionally working towards Orcus’ goals, some being duped into doing so. This required some reskinning - the generic evil cult in ‘Isle of the Abbey’ and the Lolth priest in ‘Salvage Operation’ became Orcus worshippers.
In your case, if you want to focus on the three factions of Saltmarsh, I think you could go two ways:
In either case, the political side of Saltmarsh is relatively undeveloped, giving you lots of room to work in, but also lots of work to do if that’s what you want.