Can a RoG world generate the Sunken Boat but still be incompatible with SW?

[Fledge 79]

I was hoping to merge two saved game files as I've found the Seaworthy in SW and finally managed to get the world to a good enough state of progress to merge with an earlier advanced RoG world. I don't recall when I started that but the Sunken Boat is there, and I just hammered it to get the Sea Worther anchor item. I have a Shadow Manipulator, and all the ingredients required for crafting the Seaworthy, but it doesn't show up in the magic (or any other) tab! Do you always have to start from Shipwrecked and travel back to RoG, then create a return portal? Or is it possible my RoG world could be non-compatible with SW even though it generated the Sunken Boat? I don't know why that would be, except that occasionally I do use mods (nothing game breaking...) and it's possible I originally started the world with a mod, although it doesn't warn me when I open the game and I've been playing modless for a while now. Is there any way to tell whether the worlds are compatible? It's even possible I started the RoG world before I bought SW, but if so then why does it give me the Sea Worther if I can't craft the portal? I'm very flummoxed and more than a little disappointed, as I'm out of saved game slots and merging these two files would free up a slot and let me try out a new character. Many thanks for any help!

[Fledge 79]

Aha, actually have solved this myself - apparently I needed to go from SW and it let me merge the worlds, and then I was able to craft the Seaworthy in RoG and now I have my single save file  So it would seem you can only travel one way to begin with, from SW to RoG? Clearly Klei do not want us gearing up before exploring SW...