Cars (pickups) one or two fingers from the steering wheel.

Bikes (bicycles) do this if they aren't training

Yeah but your examples are members of the same community. What is weird and endearing is that people who have never been on a boat will wave at people just because they are on a boat. Conversely, people on a boat will eave at people on land. Its as if a transport vehicle was a ride. A jet ski or something.

Once you get outback in Australia pretty much everyone will at the very least lift one finger off the streering wheel.

I like to play a game when I'm driving, if they don't wave I wait until the last second to do it and see if they're fast enough to react before we pass

Bus drivers, truck drivers, farmers do it too, established villages smaller than 150 inhabitants as well.

Small community theory: I'd argue that it's associated with this peer group of approx. 300 people thing, where humans get influenced by only a very few hundred people directly or smth lioe this, but I don't remember the source.

Whitewater kayakers: brown claw instead of wave.