The boat will probably be somewhat crowded, so forget about using a tripod (or even a monopod) for your big lens. I’m fit enough that I can handhold my 500mm lens for short stints, but it’s never easy. So consider using a 300mm f2.8 lens with a teleconverter on a smaller-sensor camera body to maximize your comparable focal length. (A 300mm lens with a 2x teleconverter on, say, a Canon APS-C body gives you an equivalent full-frame focal length of 960mm.) Either way, the best way to track a flying eagle is to handhold your camera. Use a wide-open aperture with a fast shutter speed, and vary your ISO based on the light level.