Will this work?

We always just used an empty 2 liter pop bottle. No markings. It was just kinda the universal, international symbol of a shallowly submerged rock..........floating pop bottle.

Sometimes, I like doing things the hard way.

A white Clorox bottle tied to some polypropylene, 8'... 6'... down, tied to cinder blocks, marked boulders and shoals in the small lake my parents had a summer camp on up in the Adirondacks, in the late 70's...

Richard buy a good depth finder and stay away from the rocks

Part of the idea is to mark reefs for fishing,

That is what we use at our club and a lot of other places I fish in PA.

People here tie 2 liter coke bottles to snags in the river for markers. A white bottle would be more visible.