"unauthorized" buoys

I'd like to train some slalom racing, but have no buoys to practice against. Am considering making some "unauthorized" ones - there's no way I could get permission from coast guard/lifeguard.

I considered using an empty detergent bottle tied down with cheap line to an exercise weight, but this would be kind of dangerous if struck. I certainly don't want to endanger somebody.

Any suggestions for something cheap, quick and safe - that's visible from water, but not too visible from shore that it would attract lifeguards? We get plenty of trash in water - I just want something that looks like trash, but somehow doesn't keep on drifting

Sailing in open ocean off a sandy beach. Not sure what the depth is, but it's nothing extreme.