The DS-51 will barely cover 24mm EFL u/w and adjustment may be iffy presuming you use a dual sync cord? Remember it is a one rectangular tube flash. Higher Ike model numbers get you the circular tube flash tubes which can cover WA.
IF you really want to do CFWA or just WA shooting, you should compare cost of single DS-125/160 to two DS-51s. And of course their weight.
Depends on what kind of shooting you're doing too?
I am thinking along the lines of the examples in my first post, ie. wide angle with a dash of light onto something in the foreground. I have rarely shot things where I have felt I have needed to fill the entire fram with light. (Sadly no whale sharks so far for me ) I'm using a 15mm (so it 's full frame) with 8" dome. Though again with the travel/size aspect I am considering to try the 6"