Scuba here too, and found it pretty universal, at least as universal as I traveled (Hawaii and Honduras). Most people seemed to think it was the O of OK, which is weird if it is truly universal because english really isn't. Just because someone doesn't know the norms doesn't mean it should be thrown out, it's outlined quite well in every rescue manual I've read. I'd say it's best to talk to the person who isn't following the river signal norms and try to educate.
I wonder if it goes back to military/navy. My dad always used it for lake boating growing up, he had no scuba/river training, but was in the navy on subs.
Living in Montana, boating in Idaho