I have to admit, though, my tendency toward etymological literalness makes the phrase "space marine" seem kind of oxymoronic to me, since "marine" means "of the sea" in Latin. It just seems to me that if you wanted a word for someone like a military marine, but in space -- i.e. a member of an infantry service specializing in spacecraft-based or interstellar operations -- the appropriate construct would be something like "celestine." Although I guess that doesn't sound very macho.
Just saw that. See.... internet pitchforks do work... (Of course, I'm sure someone is going to say, "You don't KNOW that the chatter on the interwebs caused them to change their mind..." Nope, I don't. But, we don't know it didn't either.) (Parse that double negative.)
In a sense, yes, but not by strict etymology. "Space navy" is fine, since "navy" comes from the word for "ship," but "marine" means "of the sea" specifically, so if you're transposing it to space, it ceases to be literal. It's also ethnocentric, since "marine" is a term used mainly by English-speaking countries. The more generic term is "naval infantry" -- which, as discussed above, would work just fine in space.
I work with a bunch of Marines at a shore command. They don't come "from the sea"; they come from Virginia. Still Marines.
^You don't need to point out the obvious. I'm not an idiot; I'm a writer and a lover of language and I'm making some observations about etymology.