Just my 2 cent's here, I dive allot with cave diver's using both 10 Watt Hid's, and 21 watt Hid's, And I have to say, using the 21 watt Hid myself , ( my Buddie's light ) , that when I did use it for the " OK " signal, I did not tighten the beam down to 6 degree's, as it is to much light, I was probably around 10 to 13 degree's , because being so bright, I found it wasn't like using a 10 watt Hid like I normally use, and turn it all the way in, to 6 degree's.
I would have to agree with Tamas here, a 10 degree light head would be just fine for signaling I think " my opinion " .
That's a nice light, I was looking to get a 21 watt Hid here soon, but I saw your 50 watt HID on Ebay, for around $1,500 , and I was thinking of trying it out. I would love to see a 50 Watt Hid in use, I'm hoping someone here buys one, so I can see their response, and the pro's and con's , weight , size of the light head, burn time's, etc, beautiful light though!!
Good luck in the USA , if it sells well, hopefully the price's will drop on HID's period