The stuff is so low in the water that it can't be picked up by radar. A
debris object would have to be nearly as large as your boat to set off any
radar collision alarm..


This hasn't been my experience. In calm inner waters, such as the inland
passage along the west coast of Canada, radar when set to short range can
commonly pick up a log or the top of a deadhead if it breaks the water's
surface, and it will routinely pick up a seagull standing on top of it. In
fact, I've even seen it pick up the riffles from a tidal rip.

Tom Dacon