In Hawaii, as in many places, a dive flag is required by law. However there are times where I have forgotten my flag, and times when it's inconvenient to carry down the beach with your arms full of other gear. I've tried clipping my flag off to my bc for the walk to the beach (not during the dive ... that's dangerous) and I've tried clipping the line off and allowing the flag to drag in the sand behind me. Neither is an elegant solution and after forgetting my flag several times I wanted a more convenient way of dealing with my flag.
In came the idea of attaching it to my SMB (submersible marker bouy) as I keep this piece of equipment in my wetsuit pocket it's always along. I removed the flag from it's "pole" and took out the stiffening rod that was inside of it. Next step was stitching it to the side of the SMB - just have to be careful not to sew it too far into the SMB, where you could puncture the air tight bladder. Now I just roll the flag up and leave it in my wetsuit pocket for use. I walk down to the water, surface swim out, and during the surface swim I pull out my flag, inflate the marker bouy and drop down. I can either tow the flag during the whole dive, anchor it at my exit point, or my favorite, I simply bring it along on the dive, deflated and rolled up in my pocket. At the end of the dive I redeploy it, inflate and send it to the surface while on my safety stop. It's always with me in case of an emergency ascent and takes less than a minute to deploy or stow. For a line I use a small finger spool with 50 feet of line and a stainless steel double ender.