... and they have no clue that you can't go outside the markers without running aground. (It's even worse when your boat has a 28 foot beam ) I have a loud-hailer on my boat that's built into the VHF radio -- I usually use it only for the automated fog horn function, but got on the PA like Rodney Dangerfield (in the yacht scene from Caddy-shack) on some kayakers once because they wouldn't leave enough room.
... Then there are the dark blue and dark green kayaks, or camo, out at dusk. No so obvious.
BTW I also have a siren function on that. It's a GREAT way to slow jet-skiers down in the harbor. A couple of quick siren "yelps" and they shut it down fast and look around. It's funny to watch.