Had a motorcycle back in mid/late 80's and all riders waved. Harley guys would hold their left fist down beside them instead of wave though. I had a Yamaha and never waved like them...I was scared they would come back, beat me up and father rape me!
We've been boating regularly since 2001/2002 and most boaters will wave if conditions permit...or one of the passengers will while the captain maintains the wheel.
If you've ever driven rural GA and probably other similar states, you always just lift your index finger off the wheel when passing another pick-up truck.
I figure with my class C overhang, most folks couldn't see me waving. But, it would be nice to recognize others you met on the road.