Like aramis said, they were larger than the army (essentially nearly every ship had a few cross trained sailors who could serve as makeshift soldiers), but weren't like the marines nor army in terms of equipment, training and overall use.
Not the army, the marines.
Naval Infantry in the US were standing additional duty assignments, per http://www.history.navy.mil/library/online/naval_infantry.htm...
Note that during WW I, Masters At Arms. Just add a roll for infantry duty to the navy tables... and when attained, resolve as a marine infantry year, and using the Marine Infantry MOS and Shore Life tables.
1920 Equipment was a rifle, leggings, Load bearing harness, haversack, blanket, shelter half, ammunition bandolier, mess kit, canteen entrenching shovel, and duty uniform (blue or white). For intended durations over 24 hours, a rubber blanket was added, and a jacket and/or second canteen might be added.
Oh, and the standard Landing Regiment was lead by a Battalion of Marines, and included two battalions of naval infantry, and one of naval artillery...
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