Adjusting Ghosts of Saltmarsh nautical rules to Spelljammer is a combination of extreme similarity and extreme difference. I'll break it down to sections.
This message was last updated by the GM at 01:31, Tue 18 Oct 2022.

Several new Background appeared in GoS:

Fisher: Fishing IS possible in the Astral Sea, but it is NOT a body of water.

Marine: replace Vehicles (Land) with Vehicles (Space) if you're a Wildspace native. Don't if you're a groundling new to space. This gives you skill in space, of course, but also with the ship while landed in a body of water, which is different. Land vehicles won't come into play much. A spelljammer can fly over the land.

Shipwright: The abilities here are limited to a WATER vehicle. If the ship isn't capable of water landings, a Shipwright can't fix it. It also needs to be made of wood, of course. In space a Mending spell does much better, anyhow. Of course, a Shipwrigt who learned Vehicles (Space) rather than (or in addition to) Vehicles(Water) CAN design and build a spelljamming ship. Carpentry is used for Wooden hulls, Blacksmith for Metal hulls.

Smuggler: If a Space native: replace Vehicles (water) with Vehicles (Space)
This message was last edited by the GM at 21:37, Sat 29 July 2023.

Upgrades cost 15000 gp and 1d4 weeks. Ships start at 20000gp, so they are pricey. But that's the GoS cost. If we convert magic items to something more in keeping with the DMG, things change.


Magical equipment:
-Most are moot/useless: any oars and sails, for instance.
-a Churning Hull requires ocean gods, so, presumably, an ocean.
-There is already another sort of Living Ship in SJ, one that isn't dependent on a steady source of water.
-Vigilant Watch Hull should give a penalty to Beholder encounters...not that you're likely to notice.

Some specific items:
Vigilant Watch: You provide the eyeballs. 10 from one beholder, all undamaged. Then someone can enchant and place them for the 15kg.

Arcane Artillery: Replace with standard magical weapon mods and costs. Ballistas cost less than a long bow. No reason they shouldn't cost the same as a crossbow to enchant (+1 through +3 and etc) Ammo can be enchanted as magic ammo. The same holds for Mangonels

Explosive rounds: cost on these is pending my finding an equivalent item or spell.

Guardian Figurehead. This will likely be pricier than GoS, as it's a golem under control of someone not it's maker.

Smuggler's Banner: Price pending

Undead mods:
Screaming Sails: TBD
Bones of Endless Toil: TBD

Variant from DMG:
Ballista Bolt: as one use Javelin of Lightning (priced as consumable item)

NEEDS UPDATING
This message was last edited by the GM at 00:34, Thu 12 Jan 2023.

GoS presupposes an NPC crew because it takes more men to run a ship at sea. The combat system there is Ship actions. For instance it only takes one action to fire a ballista because it assumes sufficient NPC crew to man the weapons fully.

This is fine on a large ship where the PCs are the officers. However, to use an Officer action one must actually QUALIFY for that position. See GoS. for what it takes to be a Bosun, First Mate, etc.

On a smaller ship, where the crew is all(or mostly) PC, the SJ system is used: It takes 3 people to arm, aim and fire a ballista every round. Four for a mangonel. Movement is done by the Helmsman. The Captain shouts orders that will likely be ignored because folks are PCs and PCs always know better... or think that someone else will do it for them, or both. I love all PC ship combat. Chaos incarnate.

Also remember, in this edition you have to be nearly bumping hulls with a ship to have it in range.

Things happen a sea. Things happen in space. Encounters reflect only some of that. Of course, not everything that happens at sea CAN happen in space. Weather, for instance, still, things DO happen, even if less frequently, so let's look at the Saltmarsh event chart:

1-3 Crew conflict
  If it's a PC crew ignore this result. No event. If it's an NPC crew, use the standard Saltmarsh rules.
4-6 Fire
  This is unlikely in space. Cooking fires and Lamps/Lanterns burn precious air. Still it's not impossible. Give it a 50% chance of not happening (no event) and if it does happen reduce the DC of the fire one level. Then find a potential cause: In no one had oil or torches in their pacts, and there is nothing in cargo, either it didn't happen (no event) or the NPC set it.
7-9 Fog
  No event...Unless the solar system you are in has something equivalent in space. Krynnspace, for instance, has two similar effects. although one is in a specific area.
10-12 Infestation
  While it's unlikely to happen in space, Jammer Leeches count. So does a vitamin deficiency. Deficiencies are only likely if supplies are getting low or your cook isn't a trained cook. If you're living on created food or goodberries and you have an NPC crew, treat this as Crew Conflict, instead. Or just let the jammer leeches eat the hull until someone remembers you should check for them occasionally (in which case CLAIM no event and wait).  No event in Astral: Even jammer leeches don't eat there.
13-20 Storm
  As per Fog: no weather in space...unless there is. If there IS something weather-like in the solar system you are in, this is a good way to see if it's been encountered.
  Note: In Astral space this does NOT signal a psychic storm or astral wind or the like. That's on the Astral Encounter chart already, or should be.
This message was last edited by the GM at 00:38, Thu 12 Jan 2023.