I think it's cute that thing humans do when they see a boat pass and the people on the boat wave at them and they wave back. For
absolutely no reason. They don't know each other they're not trying to communicate anything other than "LOOK! I am on a boat!!! Hello!!!!" "I see you!!!! On the boat!!!! Hello!!!!!!!!!!!!" in a genuine moment of wholesome human connection and excitement.
Point down with two fingers to signify "I hope you manage to keep both wheels under you, friend" ... it's very important. In the UK bikers give a little side-nod, because their free hand is on the wrong side.
When I see someone on a quad I point four fingers down :-)
Hey fair. Definitely not something I was taught. That goes out the window on sled. On sled you need to let the passerby (apparently that is the plural form?) know how many people you left behind you.