I take it that the link in the OP is the updated version?

It's updated, guess I should change that date, eh? Only the land mesh has improved though, the plug-in's still the same as the original Firemoth's.

I tried this and the companions are drowning under the boat again. I have to use reset actors to get them back on the boat. Why does that happen? Otherwise it works good, and being able to use Sea of Destiny too is great! Does this meant that Havish can be put as in interior too soon?

The drowning issue is a local reference problem -- I've addressed this in the official release I posted to Planet Elder Scrolls. I've moved the crew off to the side of the dock, and placed them on dry land. This will fix a couple of the local reference issues, like the doubling of the gates, the crew drowning, and the issue with boat placement and other plug-ins that affect the Census and Excise dock. It should be approved in a couple days. ^_^

As for other landmasses as interiors, technically it's possible. It relies a lot on how many references are present in the combined cells, as well as any scripting involved. :shrug:

Official release: http://www.gamesas.com/bgsforums/index.php?showtopic=1077863