While we are at this topic: What's speaking against having one Geoscape? I mean, the TFTD-Geoscape has the problem, that the land masses really look boring. On the other side, it's pretty much the same with the water parts in x-com 1. I'd love to see a mod, that combines the best of the two worlds. The nice land masses of UFO and the sea looks of TFTD. Would that be possible?
I've just thought about this and the answer is no, unless you have a customized .exe
It would completely possible to add the sea textures to UFO's Geoscape (or vice-versa) but if you shot a UFO over the oceans the crash site would still be generated and it would be possible to assault it. That's huge problem #1 and I suspect it wouldn't be the only one.
Of course, it wouldn't be an issue if you were running a hybrid UFO-TFTD game but that poses a lot of other challenges as well. Basically you'd either have land or sea bases, but not both, and to keep things more or less sane regarding plotline and consistency it would make more sense to use TFTD. Tritons operate on land while Skyrangers and other planes aren't designed to submerge, along with other minor things. And other issues that I can't remember right now, but yeah, there should be quite a few.
So possible? Yeah, if you're a TFTD fanatic who is willing to spend an great amount of time (and frustration) while working on it