Given that buoys were preceeded by withies, which are tree branches, or boughs, could there be linkage there?
Especially as bough seems to be an older and specific word in this context, compared with branch which is a without-portfolio johnny-come-lately racey French import.
I can go with the Dutch derivation but not the German one. Next they'll be offering a word they've back-formed into proto-indo-European meaning 'floating log'. In general you have to watch those dicshunary wallahs and their unwarranted extrapolations - sorry derivations. For example, they give fan (as in football fan) as derived from 'fanatic'. Hm. But the followers of sports in England in the early 19th century were called 'the Fancy'. So what's that, just a co-incidence?
The first recorded buoys were used by mariners in the Guadalquivir River, approaching Sevilla, Spain back in the 13th centurary. There's a bit of history for you...!
OK so that was when the Moors were still in charge .. at least in the South.. so my theory that Buoy is moorish / berber for Crusader's Head is true.. lol
We also pronounce it 'booie' in the NW of Scotland. Maybe the N Americans got it from us (as so many went from here to there in the 18th and 19th centuries).