Some Possible Solutions (that may cause you more problems):
1. Document the offending drone use by date with video (use your phone) that shows multiple incidents and the distance from your boat, distance from the land/home and the duration of their buzzing of you. Save this for evidence.
2. Make a poster or large sign that asks them to stop buzzing your boat. Make this poster easy to read and make sure it is pointed at the drone if it is close to your boat. Show this to the drone. Document (with your own video) that you are politely asking them to stop. If the buzzing or harassment continues, change the sign to a warning sign. Document this too.
3. Notify the authorities, using your video evidence of multiple times it has buzzed you, to document harassment. Show them the video documentation
and your written records of multiple drone close buzzing incidents. Get this report documented.
Ask local police department what you should do. Document the LEO answer on video. Get the quote from someone in a management role. Do the same at the Mayors office.
4. If the offensive activity does not stop, or if the authorities claim they are powerless to help consider next steps.
5. Using non-lethal, (no guns) and legal
items (fishing rod, sling shot) devise a way to disable the drone if it is buzzing you, but only after you have asked them to stop.
A couple of folks mentioned the wrist rocket sling shot.
That would be my first choice IF the drone is very close to the boat. If it is not close to the boat, I don't think you should be so bothered by the drone, and I would just ignore it.
For disabling the drone that is flying very close and low, I think a sling shot with a "bolo" type device such as two lead fishing
weights joined by meter long length of leader wire would be my choice. The goal would be to entangle one or more of the drone's props, causing it to fall to the water. As the linked video showed, monofilament fishing line may not stop the rotors, as the rotors could cut the mono.
Is it illegal to down a privately owned drone over the water, after it has been established and documented that you were being harassed by it, after you attempted to communicate with the drone owner by posting
a warning, and that you went to the proper authorities to try to stop the harassment?
I think the key here is to carefully document (video and dated) the numerous times the drone has buzzed your boat, your attempt to communicate with the drone owner (fair warning was given via the big poster or sign ), your attempts to go through local law enforcement, how you were advised by local officials, and then your own attempt to stop the harassment while using non-lethal, legal
devices or tools.
However, I would expect that any boater that causes an expensive drone to be downed and lost
in the water will likely cause the offending drone pilot to seek retaliation for his loss. Whether through legal means or mischief (vandalizing your boat or dinghy
while you are away) it could prove costly.