What Mike said… I know when I dive, I can easily be in any position relative to gravity, and remain there for some time. Especially when I do cage-less shark dives, motion can attract attention and that is a bad thing, especially if you look tasty. The man down alarms we use in emergency services will go off every 30 seconds or so even if you’re just standing still, you need to frequently and repeatedly swat the thing to keep it from going off. I don’t see this being practical nor a good idea in a dive situation, as the ‘proper’ dive position is usually with your arms crossed in front of you.
Under what scenarios are we diving in?
If we’re in a situation where direct contact from your dive buddy is not practical (cave or wreck diving), then you should be using radio comms to maintain contact. In these scenarios, a blinking light is pointless as the victim/diver is already obscured by whatever was preventing you from maintaining line of sight contact. That’s why we use ropes/tethers.
If we’re in an open water situation, there’s no excuse for not being close enough to your dive buddy to see if they are in distress. And again, if the water is cloudy enough that you can’t see your buddy, then a light will not be that effective. That said, buddies do drift away sometimes, chasing giant turtles or whatever. But when that happens, it should only be momentary, and the pair should reunite ASAP.
With everything involved with a dive, I think a positional trigger will either be always going off, or never going off. The motion of the water, or the movement caused by an unconscious body as it slowly settles to neutral buoyancy will prevent the sensor from tripping. On the flip side, if those are accounted for, the body would have to settle out before it trips, and by then it could be too late.
I think, if your intent on building such a device, you need to change your mechanism. A couple of ideas, some practical, some not:
Monitor the air flow and breathing rate. Only the most fit divers would have air consumption rates near their unconscious levels while alert, even then a couple quick puffs on the regulator would reset the counter.
Add a pressure plate to the mouthpiece. An unconscious diver would relax their mouth, and then the counter would start. If they’re just a calm diver, a quick lite chomp would reset the timer.
Anything you build, though, would need an auditory warning before the alarm is triggered. Firefighters have a couple few chirps from their alarms before it goes crazy. An audible chirp underwater would be very noticeable by the diver, and they could easily perform the reset action. Sound carries a looong way underwater, so you wouldn’t need a powerful source to alert the wearer.