I've got to go with Robert's suggestion of "more attentive buddies" at the top of the list.
The 18 and 21-watt canister lights at are pretty effective, but only if the buddy is paying attention. That's the next piece of gear I'll purchase. I've had the pleasure of borrowing one and its pretty amazing.
I would compare these lights in the context of a night dive as follows when it comes to actually being able to see them as a signalling device. Most of us have seen the difference between a standard dive light like a UK C-8 compared to a UK Light Canon. The Light Canon makes the other light's output look somewhat insignificant. This is what a 10-watt HID can light makes the Light Canon look like. Continuing the comparison along the same lines, this is what an 18 or 21W can light makes the 10-watt can light look like. I find the difference between an 18-watt and 21-watt light visible, but not anything like the difference I've mentioned previously between the other lights.
During the day, I find lights worthless until you get to a 10-watt HID can light. Then the difference between that and an 18 or 21-watt is pretty significant. The same 18-watt can light with an EKPP reflector was clearly more effective during the day than one without the EKPP reflector. At night, the two reflectors were comparable for signalling purposes.
I hope this helps put "lights as signalling devices" in a slightly different perspective.
While the quacking of the air horns is quite annoying to others in the water if used for the simple, "Hey, look at what I found", it would be handy for getting attention in an emergency. This would be expecially true if you were partnered with someone you were unfamiliar with as a buddy.
I'd rather go on a simple shallow dive with someone who is unfamiliar to me before doing a deeper dive with that person. In many cases, I find that even if the person is attentive and skilled as a buddy, it helps to have some time to "get in sync".
Christian
