Okay, I've found a new answer to the "cheap buoy marker" question.
There are a number of items often given away at the gates of sporting events (football, basketball, etc...) called "thunder sticks" or "bam bams". They're simple plastic tubes about two feet long and usually imprinted with a team's name. They're provided in pairs. The idea is to unroll the tubes (like a mini-safety sausage), blow them up, and then "clap" with them, producing a ringing percussionary noise.
Okay, I obtained a supply from my local high school. First, I glued a strip of plastic near the mouth of the thing, then attached a length of cord (about 80 feet). On subsequent attempts, I simply slit the excess plastic material near the mouthpiece and tied a loop of cord through it. It holds extremely well.
To deploy the device, a small puff of air is blown into it. Remember to use only a partial breath...there is no safety valve to release overpressure as it ascends to the surface. Allow room for expansion. To deflate, simply insert a drinking straw into the nozzle several inches (to get past the simple, plastic leaf valve inside the tube). It rolls into a very small package (about the size of a light stick), so several could be carried at a time in even a small BCD pocket. There are pictures of one of the first designs in my gallery.
It takes a bit of practice to deploy at depth, since you have to remove your reg to blow into it. Some interprising and creative souls could probably figure a way to inflate it with an octo. Still, it's an option worth investigating.
