At first glance, this looks like ice floes, but it was not. A quick probe with a boat hook showed that it was actually slush cakes.
And it was stable - the snow "floes" lasted for more than an hour.
That's my theory anyway. Maybe one of you from a cold weather port can substantiate it, or debunk it.
And why am I posting this just now? Because winter has returned to Seattle, at least for now.