http://tinyurl.com/l28slc

Dave Nitsche need not read.

Basic! At least for one as old as me...

Well I could always read a flash chart and do the calculations - but never bothered to understand the genesis of them - until I decided to do flash photography and without the benefit of an LCD, I thought, I had better KNOW how it works as opposed to how to use it and calculate it...

I use it all the time, I don't like TTL to decide for me. And underwater it is hard to do without. You have to divide the GN by 2 at close distances. At long distances, forget about flash if you don't have a bunch of assistants to swim around with slave strobes.

Div by 2 - is that just because of the properties of water and the rule is then different than with air? I also find that fog puts off my light meter too - and I guess foggy air could also affect flash performance too?

It is because of the properties of water, and divide by 2 is only correct for very short distances and good visibility.