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
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.
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)