As for motorboats, cars, etc, which aren't necessarily as challenging as aircraft, (the most I've done there is like out the window of a puddle-jumper where I was just a passenger) ...say you don't have fancy stuff.... your body is good.
Relax somewhat more than you might otherwise, let your elbows come out a bit, let the camera get off your face, breathe low as usual, don't tense up. (as is natural and sometimes rather sensible, depending how the boat is going.)
It's certainly not equal to any kind of purpose-made tech, but your body is actually a lot better at damping vibrations than imitating a tripod as usual might lead you to think. Things that make you steadier on a solid surface may transfer vibration I'd also suggest letting notions of a level horizon go, if the boat is moving. Frame a bit looser and fix it later.