When I was a kid, back in the '50's and 60's, Dad had an 80 ft. sailboat and when we hit bluewater I was allowed to haul out my .22 rifle and shoot at collected trash.
Shooting from a moving vessel at another moving target definitely takes some skill. As far as a spotter, no way, no use other than range and you will NEVER make a distance shot under these conditions.
If you really want to get into details, I have them. Wave lengths/heights, ship roll rates. In the end it all goes back to instinct shooting which is what Dad was teaching me.
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