I've seen lots of cases of unsafe boating. Some by boats that I've been running. (Oops).
I had a ski boat pulling a young girl cut in front of our sailboat as we were motoring at 6 knots. The skier crossed our bow less than a boat length away and this was after I had cut power and turned away.
I didn't do anything. The driver (looks like the father of the skier) seemed to be surprised when he looked back and saw me. Being a sailboat, I didn't have a handy horn to blow (he was long gone by the time I could have gone below and gotten the air horn out). I didn't bother with the radio (yes mine was on, monitored, and my remote mic was at hand) since I doubted he was monitoring and likely couldn't have heard it over the noise of the water and motor. No sense in filing a complaint with the Sheriff as they would be hard pressed to be able to take action without witnessing the event. My feeling, based on the surprised look on the father's face, is that it likely scared him more than it alarmed me. Hopefully this was a good learning experience and he will be more careful in the future.
I've seen plenty of close passes by large boats throwing huge wakes. This does create a safety issue for the waked boat. I consider it discourteous in the extreme, dangerous, and in some cases illegal. However, there really isn't any good recourse. I've never broadcast a complaint on the radio as when I hear that type of thing it is obvious that it is ineffective. If I sustained significant damage or injury, then I would contact the USCG but that has never been the case.
We always seen kayak and SUP renters in my harbor who are doing stupid stuff. Primarily it is not having a PFD and often sitting in the middle of a narrow channel while draft limited boats are trying to get by. They also have a bad habit of suddenly turning into the path of large boats. Fortunately, the harbor is one of the best protected harbors in Puget Sound and the water is generally very quite. The harbor is also a no wake zone with a 5 mph max speed. Most of the resident boaters are used to these weekend water bugs and are very cautious. The only time I have done anything about it was a few weeks ago when I was being towed back to my slip. The Vessel Assist boat was attached to my stern quarter and we had limited maneuverability and the Vessel Assist captain had limited visibility. We both use our horns to warn and scatter these folks.
I have considered discussing the rental problems with the three primary rental operators to see if there is some additional guidance they can give to their customers. I am happy to have these renters enjoying the water. They help support the economy of Gig Harbor. I just want the to be safe and to not endanger other boats.
That should be enough to get you started....