Marines aboard ship
This question has been asked -- and answered -- several times. Of course, the problem with an electronic forum is that once the post ages off, it might as well never have been written for people looking for answers. So I'll give it another shot.
First, it is a mistake to compare Honorverse ships to modern wet-navy ships. They are far more akin to the sailing ships of the late 18th through mid-19th centuries, especially on "peacetime" deployments. They operate on long, remote deployments; they frequently operate alone; and they are tasked with a multitude of responsibilities, including commerce protection, pirate apprehension (where possible), planet-side peacekeeping duties, etc.
Second, Marines are designed (among other things) to provide the security detachment aboard ship, to board other ships where necessary (as, for example, in retaking a pirate's prize without blowing it up), provide the planet-side intervention forces needed for peacekeeping, and like that there.
Third, at no point has it ever been stated or implied that Marines aboard warships in the Honorverse have no shipboard function in combat. Indeed, the opposite has been stated on at least one or two occasions, in the form of references to specific weapons mounts manned by the ships' Marines. As an analogy, the number 3 turret -- "Q" turret -- of HMS Lion at Jutland had a completely Marine crew, and it was the turret officer (a Royal Marine, Major Harvey) who saved the ship from destruction by ordering the turret magazine flooded after he himself had been mortally wounded by the hit which disabled the mount. The same practice is followed in navies such as the RMN. They also assist in damage control and any number of other functions when the fit hits the shan.
Basically, it would be a gross mistake to think of the Marines as anything but elite troops. Not only are they trained for ground and personal combat, but they are also trained for sophisticated shipboard duties. They are certainly not wasted manpower. And I do not think casualties among Marines have been heavier than among regular naval personnel in most of the battles Honor has fought. Since they tend to be concentrated on just a few weapons mounts, the casualty rates will be higher if those particular mounts are hit, of course, but any pattern of higher losses among them in the books is only apparent, not real. Or, at any rate, accidental if it actually exists.