I understand what you are saying about scootering bad vis, as I have done it. However, after such dives, I always rethink about more scooter dives in such a condition because...
If the viz is so bad, can you see where you are going in the first place with a fast scooter ? And can you see anything (i.e. wreck, reef) while scootering in bad viz ?
When the viz is bad, swim is usually better as you could see more.
Unless you are scootering in bad viz with no reference point/surface, then I will
and
, but I tend to scooter closer to the seabed or side of the wreck/reef when the viz is limited. Then a simple flashing of the light beam against this reference surface should get your buddy attention.
Remember like when driving cars in convoy, it is not the responsibility of the person from behind to keep up, but for the person leading to make sure everyone behind is still within contact distance.
The person who is following simply shines his light in the sight zone of the leading person to let him/her know that s/he is still there. As soon as the leading person do not see the light spot, s/he should check.
Anyway, I can't find the exact horn used, but they were all different versions of this type here...
http://www.aquatecusa.com/ah-150.htm
They make duck-like sounds u/w, not really loud that you could hear immediately over the scooter (also depending on the type of scooter as well, Zep - forget it). I am pretty anal about my gear, but never managed to keep mine working for more than a few months when I was using them years ago.
Good luck...