This rather serious report should be of interest to LW. It argues that autonomous robotic weapons which can kill a human without an explicit command from a human operator ("Human-Out-Of-The-Loop" weapons) should be banned, at an international level.
(http://www.hrw.org/reports/2012/11/19/losing-humanity-0)
I suspect people are mostly just scared of killer robots because they're new. At least, they're scared of the new ones. Land mines are generally considered a problem largely because they stay there after the war is over, not because they kill indiscriminately during the war, though that seems to be a problem too. I haven't heard of naval mines being a problem at all
Naval mines aren't intended to kill by surprise, but to deny access to a shipping lane, shut down a harbor, or the like — nations declare when they are mining a stretch of water.
Also, kids don't run around in deep-water harbors and shipping lanes the way they do in fields that might be land-mined. Access to places that might have naval mines in them is largely restricted to vessels with adult, professional crew.