Assume the vis is 50-60' near the surface - pretty normal for around here, and I suspect for a lot of other "boat diving" places as well. (The facts get a LOT worse if the vis is not this good; I'm being nice here!)
Assume that at idle, this dive boat travels at 6mph (~5 knots or so, which is about right for most twin-screw sportfish-style - or dive-style - boats) "Monkey boats" - small vessels - can idle at 1 or 2 knots. Larger boats, particularly diesel ones, DO NOT!
Therefore, this boat travels about 500 feet in a minute.
In six seconds it therefore travels about 50', again, at idle.
There is NO WAY you are going to stop 50,000 lbs (what my boat masses) in 50', and besides, 50' is about its length, which means I'm hosed as you're under the pulpit (and thus invisible) by the time I know I'm about to run you over in that next six seconds. The SHIFT TIME ALONE from one direction to the other for mechanical transmissions is in the 2-3 second range; if the transmissions and throttles are electronically controlled add another second or two for the interlocks and verification that the builders put into the gears to prevent a shift while the engines are at speed. Then add the fact that it takes thrust over time to change a velocity vector; it is NOT instantaneous (boats follow Newton's laws, more or less!) Bluntly: If you're under the pulpit or behind that point while I'm in gear, you're almost unavoidably (both to me and you) chum.
Now, assume that all boats honor your dive flag, and travel at no more than idle speed when within 300' of it (ha!)
Further assume that these boats all draw about 5' (reasonable for a 40ish foot dive boat), and that at some critical distance below that you are inexorably drawn into the propellors if the boat is under way (say, another 2-3'.)
Therefore, if you are in a zone from 7-8' to the surface, you will be drawn into the propellors and killed if you are directly in line with said boat.
The question becomes, can you AVOID this danger as a diver, and is your ascent posture material to your risk.
Reality: AT BEST you will have six seconds of visual warning before you get run over, IF you are looking in the correct direction at the time the danger becomes apparent. Since you don't have eyes in the back of your head, and your peripheral vision is probably no better than 135-150 degrees, the odds of this are at best one in two to one in three. Remember, once you get within the last 40-50' of my screws and in a plane with them, if I'm in gear you're cooked.
Reality: A close encounter with a large vessel's driveline is very likely - probably in the 90%+ range - to cause a fatal injury. Even at idle you are no match for a vessel's driveline. I have run over a forgotten trail line at idle, and the driveline shredded that 10,000+ lb test 3/4" line (it was one of my travelling docklines rated for storm conditions!) like it was butter. I didn't even know I had done it until I tried to get on plane and noticed a significant disparity in speed (due to the line wrapped around the prop and shaft!) It is highly like that if you are run over, other than the sudden appearance of chum behind the boat the operator would be oblivious to what has happened (unless your tank was breached - then he'd definitely know!)
Reality: You will hear the engines of a boat long before you see the boat, but you will be unable to localize the bearing due to the difference in transmission speed of sound underwater. I hear boats from the bottom in 100' of water all the time - I just can't figure out exactly where they are.
Reality: The boat you see directly above you, but moving, is not dangerous, since it moves 50 feet in six seconds and as such will not be where it can chop you up by the time you get there. Ditto for the boat that is directly above you but with the transmissions actually disengaged, even if the engines are running (the danger is not the engines, its the drivelines!) [Side note - many boat's transmissions - including mine - sometimes "creep" when out of gear despite "neutral" being selected - you can get chopped up ascending under a running boat even if it is out of gear! NEVER, NEVER, NEVER ascend under a boat that has running engines, even if you KNOW the operator is aware of your presence and will NOT engage the gears! Someone died down here last summer in exactly this manner ascending under a boat and attempting to grab one of the shafts to steady themselves while working on some kind of problem. The driveline crept forward and he was killed by the propellor; the captain never touched the gearshift lever.]
Reality: At a "final" ascent rate of 30fpm from a safety or final deco stop at 15-20', it will take you anywhere from five to eight six-second danger periods to make that final ascent. During the first two to four such periods a boat that approaches will pass over you without doing you harm, except possibly from you soiling your wet or dry suit.
Reality: During the terminal phase of your ascent, the last three to four six-second periods where you are in actual danger, from the time you see the danger you have six seconds to do something about it. It is extremely unlikely that you can both arrest the ascent and get below the danger zone in the intervening six seconds, and this likelihood decreases in direct proportion to (1) how close you are to the surface, and (2) the number of degrees you must rotate your body to swim downward, as there will be no time to attempt to dump air from your BC or use breath control and wait for passive buoyancy effects to help you. At some point before the entire six seconds pass you enter the vortex of the hull and propellors and nothing you do from that point onward matters; you will be shark chum.
There is no valid argument to be made that a "head's up" ascent is "safer" from boat propellor impacts. None. There is likely no argument to be made that your ascent posture is material to the risk of getting run over, unless you're diving in gin-clear water where visibility is in excess of 100', and perhaps even then. I'm not at all certain that even 12 seconds (~100' vis) warning would be material; can you arrest an ascent AND get below 10-12' within six to eight seconds? Maybe, if you're close to 10'. Probably not if you're at 3-4'.
I am well aware that agencies "teach" this "fact" during OW classes, because mine did. I raised the above facts to the instructor, who had little choice to agree with my logic but insisted that she was bound by what was in the almighty manual. I hate false senses of security and thought it was my duty to alert the class that the purported danger was present with or without a "heads up" ascent profile....
The simple fact of the matter is that the vessel operator in a live-boat or drift dive is entirely responsible for knowing where the diver(s) are because the diver(s) have next to no defense against getting hit by a moving surface vessel. The vessel operator can only do this if he can see either a marker or your bubbles. Your only real defense against getting run over is to send up a marker (lift bag, etc) while you are safely below the surface, certainly from at least 20-30', and to ascend on that marker's line. The boat operator's responsibility is to insure that he does not approach the marker (within 100' or so of it) until the divers are ON THE SURFACE and he can ascertain EXACTLY where they are (remember, marker lines are not always completely vertical if there is a surface current!), and once they ARE on the surface he must approach no closer than is safe for the size and type of vessel before disengaging propulsion and, preferrably, shutting down the mains (there is also a potential exhaust gas hazard with boats if the diver(s) remove the regs from their mouths prior to reboarding)
