That said, if it were my sorry butt on the line it certainly wouldn't be with an M-16 with iron sights and 30-round magazines. Range is extremely deceptive at sea, with no frame of reference to help the eye scale. What looks like you could reach out and touch the other fellow turns out to be 2000 yards. The length of your own ship might be up to 400 yards if it's one of the larger container ships. That isn't a typo, and it isn't small arms range from a rolling platform. Maybe if you're a SEAL. I'm not.
I know this from experience. My first commissioned billet in the Navy was gunnery officer and I was later an instructor in that field at FLETRAGRU San Diego. The Navy has all sorts of expensive alternatives that spit enough lead to make walking on the deck hazardous from spent shell casings. The merchant fleet isn't going to go that way, both from expense and lack of a crew trained in the field. But a couple of these guys port and starboard:
...provide a very nice standoff capability for even a modest sized cargo ship. Yep, the venerable Ma Deuce. My personal view is that if you have to go to small arms, they're too close. Just my $0.02.