Tag on the bouy says "If you can read this, I can see you in my scope!".
Or razor blades embedded in the line about four feet under the surface.
Put a zip tie on the door, keeps the slightly dishonest people out. I've had mine cleaned out a couple of times but not stolen, luckily. If you have a big pink bouy or something else that makes it stand out, it might not get hit as easily as it would be easier to see from a distance if someone was messing with it.