F22?
I'm running a PnS with a minimum aperture of F8, hardly infinite focal distance. No matter what internet dictionary you'd like to pull the info from

I find f2.8 to have far too shallow a DOF to pull any sharpness from a WA shot. F5.6 is a bit better, f7.1, even more so, and f8 being the best I can pull. An out of focus shot is not better than a grainy shot.
I don't think I've ever been able to use a 1/500 shutter speed in our waters(including spring water with 200' visibility - albeit lots of shadows from surrounding forest), WA or not, wide open aperture or not, internal flash or not. Not an option for this area in available light. I'm lucky to get 1/125 on any occasion.
Increasing the ISO isn't my first choice to gather light, but its a viable one. If I can't reduce the shutter speed enough and a larger aperture isn't conducive to the shot I want, why not raise the ISO? It gets the shot done! I'm using a PnS. A PnS thats horrendous with noise anything greater than ISO100. Reduced size images significantly decrease visible noise. There are quite a few programs available to reduce noise. Noise isn't as noticable when printed.
DSLRs handle noise MUCH better. Talk to a wedding photographer, ask if they shoot at the lowest ISO in dark churches, with or without external lighting. There job is dependant on quality pictures and includes large prints. I just don't see why the big fuss, quit pixel peeping and look at the big picture.
I'm not too worried about a strobe, I have plenty of others things for my money to goto(lenses for my dSLRs which are not housed, a new car, ect) and I'm not positively convinced I want to tote that much camera around with me UW.
There certainly IS more than one way to skin a cat.