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I get the impression you are talking about a flag that sticks up high when paddling. In another thread about visibility, I mentioned that I sometimes put a fluorescent yellow construction vest over my PFD when I really need to worry about being seen. That won't be as high as a flag, but I'm sure it will be high enough and I'm sure it will be easier to see. I was thinking about that the other day when I spotted someone doing roadside work about a mile up the road when I was driving. I wasn't even looking that far ahead at that moment but I spotted him instantly. I'd put more faith in a high-visibility vest than a flag.