NONE. And here's why...
We were in Florida for a 2 week trip, when one of our "boys" got separated from the boat. The whole time he was about 1/2 mile away but we couldn't' see him. We circled the area for quite awhile, gazing out on the horizon for his SMB. Saw nothing. Everything we saw floating on the surface that looked like a diver ended up being a buoy, trash, nothing, or a lobster pot float.
We eventually got low on fuel looking for him and was just about to call the USCG so we'd have enough fuel to get back to land when we finally saw him.
He had his SMB out, and was blowing on his whistle the whole time. He said he just floated there, watching us drive all around him. There was nothing he could do to make us know he was nearby. SMB's cant' be seen, a whistle is worthless, and the little mirror looks just like all the other 263,913,287,442,286,876 sparkles on the water on a sunny day. There were three others on the boat and all three of us failed to see him.
After that incident and almost having to leave him and call USCG, we went to a red smoke signal that is waterproof and submersible. It's cool-burn and can be held in the hand and will float and burn on the surface of the water. Now we all carry one in our BCD pocket. Mine has been to 112 feet over the last 3 months and 8 dives and is still watertight.
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