I have to agree, I find the idea of marines in Starfleet a bit OTT. It has been established that there are groundtroops, we've seen them in DS9. But we've never seen them assigned to any ship as part of the standard crew compliment. Somehow, it just doesnt fit for me. And, as said, it's been established that the security force on a starship fills the role that marines would in this day and age.