Accidents on the water do happen.

It should be ( or already is ) a special hand sign or signal that person needs immediate help.
Something 100% recognizable as SOS on the water not a greeting, or something else.

Some gesture that could be recognized by anybody as need for immediate help.

Watching this video is the obvious reminder.

I did such arrangements within my own windsurfing family but there could be nice to have a universal signs, nobody could misunderstood.