Many a mod has both underwater and land missions.
I haven't played it, but Ethereal's X-Com:
Terror From the Unknown is supposed to be a combination of UFO and TFTD.
There's another that's quite 'hybrid', but that one's a bit of
persona non grata around here, and it's just so much more (and a mess) than merely UFO+TFTD that it's no more a 'combination' of them than anything else.
Otherwise, depends on how much value you place on the underwater aspect, the 'X-Com vibe', etc. Underwater, TWoTS. X-Com, X-Com Files is popular and probably the most expansive of the variations on the original. I think there have been various smaller attempts at a hybrid mod, but don't recall offhand if any of them reached a passably playable state.