Humans rarely use new words when old words will do. When we were looking for a word to use for "thing that keeps us alive in space", "ship" was the obvious one since we already had a word for "thing that keeps us alive on the ocean".
When we are looking for a word for "soldiers that accompany ships in space", "marines" is almost certainly the word that is going to be used.
The sci fi tradition of calling a fleet of combat starships a "navy" leads to the soldiers who deploy from those ships being called "marines". It makes a certain kind of sense.
The term marine implies a soldier who specialises in sea based warfare or deploying from sea.