This must have been a reoccurring theme for soldiers over there? Although not on a boat crew, my father in law was in a Cav Unit in 68 and said he carried a carbine with him in his tank or APC depending what he was driving at the time. Said it just fit better inside the small space. Claimed he found it early on at some point from a suspected VC, tried to take it home after his tour was done and had it in his bag, but when asked before leaving, he didn't want to get in trouble and fessed up to it being there. Said they told him it was "government property" and subsequently took it.
One of these days I'd like to get a decent shooter grade carbine for him so he can shoot it in his remaining years. I don't think he's used one since he left and he'll be 70 years old in a couple months.