You mean like a surface marker, you could make a buoy out of one. Actually come to think of it you could probably have one that you send up from the bottom so the boat knows you are coming up, you could probably call that one a delayed surface marker buoy.
I recon if you send one up a good few minutes before surfacing, say 15 mins or so, it would really minimise the chances of a boat hitting you on the head when you finally reach the surface
Can I expand on that?
They could come in different colours e.g. red, orange, yellow (even red one side and yellow on the other) black or silver. The silver one could be mirror finish (useful for divers, so inclined, to do their makeup prior to the boat picking them up).