Personally, I think alot of boaters that are on the reefs we dive on, will never see the SMB, and even if they did, they will not "process" this...So they will always be running nearby smb's or floats of divers---Unless, there is a Dive Charter Boat policing your dive area, and warding off any approaching boats--even playing chicken if need be.
I would not consider sending up an SMB with the purpose of protecting me from an oncoming boat.....I do that by stopping at 10 feet...and listening.....and listening, and when I think the coast is clear, I do a rapid ascent from ten feet while being negatively buoyant( swim up ) ---and doing a 360 spin as I go, and on surface an immediate 360 scan. If there is nothing, I will wave up Sandra and Bill or who ever else I am with....If a boat is approaching and is not looking or seeing me, I am back down to ten feet in a few heart beats.
Normally we tow a torpedo float with a flag...but even with this( far superior to an SMB) I do the rapid spin the last 10 feet if the dive boat is not likely to be nearby--and if I have been hearing lots of boat traffic.
I would use an smb IF I was doing a deep wreck without a torpedo, and I wanted to do some deco after leaving it.....so the boat I was diving from would know where to look for us--that is the real function of an smb...Not for protection from stupid boaters....
An inflatable Kayak or hard shell kayak can be towed, and most ignorant boaters will see the kayak, and be afraid to run over the kayak due to the damage they would mentally process as likely if they ran over the kayak--$1000 or more in props, etc. The dive flag on the kayak they will probably miss, unless they slow down off plane when they approach the kayak.
If you have ever captained a 30 to 50 foot boat, and been cruising along at 25 to 35 mph along a reef line--you know that dive flags are NOT seen, and SMB's are even more impossible to expect the boat to see.
You need a good captain and a boat protecting your dive area. Teaching that SMB's do this is bad logic and will create a false sense of security.