I really do hope we get boarding action at some point, but doubt it will be near term. I don't know about "Marines" though; maybe the navy forces would send these over when they pull CMDRs from supercruise for random military inspections.
I think CMDRs would hire "Specialists," "Security Officers," or "Private Contractors" to fill the equivalent role. They wouldn't be marines, but more like paid mercenaries or even partners to the CMDR (depending on ship size, number of security personnel needed). Obviously CMDRs should be able to take boarding/defensive roles upon themselves also.
It would make a lot of sense to have some combat androids/robots also (for CMDRs with a lot of credits), and probably automated defenses too. (Turrets, gas, electrified corridors, barricades, doors, explosives, etc.) Not all CMDRs would want to put the defense of their ships into someone else's hands.