As far as I know (and have observed) the NPCs will only have access to the vanilla submarines, typically reversed left to right. I've made several submarines and added them to the Workshop, I've never seen any of these or the custom sub I'm sailing in being used by NPCs.

I've heard mixed problems with editing save files. If done cleanly (e.g. by a good save editor tool, or correctly by hand) then I see no reason why it shouldn't work flawlessly, I don't think the game does anything sneaky, all it will probably care about is correct xml file syntax and filepaths etc.

Always keep a backup of your save file before making any changes if you don't want to risk losing progress.

https://ignis05.github.io/baro-save-editor/
"EDITOR"
Suspicions were aroused by one mod (also for a submarine), the description said "to make the game more interesting, I allowed the npc to use this submarine too." I've only seen this in that mod (name: Explorer).

And while you're still here, Will there be any problems if I add a submarine from the workshop through the editor, but do not activate the set in the game (I added the submarine file to the walkthrough, but did not activate the original mod)

I think once a submarine is in your save game file it doesn't matter if it's activated as a mod in the game, as the submarine is in your save file, they are two separate entities. The submarine in your save game is an unique instance and will not be influenced by any changes made to a submarine mod, if you get my meaning. It's like when you buy a custom sub and use it, if you then change that custom sub in the sub editor and add that custom sub to the game as a mod, it will not show those changes in the custom sub you are currently using (you would need to buy the sub again in-game, or edit the save file again to replace your current sub). Just be wary as if you do this then you would lose any in-game contents or changes made to the old sub (unless you edit those in too).

I hope that makes sense!

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NPCs (Pirates) will only use the same sub if the sub creator has created a pirate version of the sub for them to use. It also won't be a copy of your sub, they will have their own equipment and armaments.

Originally posted by Medium B*t:

NPCs (Pirates) will only use the same sub if the sub creator has created a pirate version of the sub for them to use. It also won't be a copy of your sub, they will have their own equipment and armaments.

Thanks