What is the optimal nozzle configuration (if any) for a pyronol torch intended 
for underwater steel cutting at a depth of up to 140' of salt water and a 
material thickness  of up to 3" of steel?

Perhaps you might want to look at alternative torches?

://www.google.com/search?q=underwater+torch

gave

http://www.petrogen.com/underwater.html
http://www.magnumusa.com/
http://www.ccmr.cornell.edu/ask/Lee.htm

Not to mention
http://directory.google.com/Top/Business/Industries/Manufacturing/Machinery_and_
Tools/Welding_Equipment/?tc=1

I'm interested in a cutting tool that doesn't require surface support-
Hoses, tanks etc. Somnething a diver can use without being tethered
to a support vessel.

try here

http://www-iwse.eng.ohio-state.edu/ouwc/

from my experience I would suggest that you would be better off using
the BROCCO cutting system (ultra thermic lance) this device comprised
of magnesium welding rod and high pressure O2 burns at over ten
thousand degrees fahrenheit and would make short work of any 3" steel
you need to remove.
The system was originally devised by the US navy and was first
utilised during the salvage of the wreckage from the pearl harbour
aftermath (WW2).
I have found it to be a most efficient demolition tool...

Re: burtrow
as far as self contained underwater cutting tools go there's always
the good old pneumatic cutter (angle grinder etc) attached to a
compressed air cylinder or if you are familiar with the origins of
underwater welding you will know of the original gas variety using
hydrogen oxygen and accetalene, ofcoarse it can be rather cumbersome
lugging several extra tanks down with you and you would ofcoarse be
restricted by the length of the torch hose any way but atleast this
way you still have some added mobility whilst at depth...