Funnily enough I did the same in Egypt last year, after completing my Scuba Diver in Mexico 2 yrs ago, decided to take a quick holiday to finish my Open water, then do 5-6 more dives. As it happened the Divemaster suggested i do the Advanced open instead of just normal dives, worked out a similar price to be honest and gave me an insight into some of the specialties available, including a night dive, which is a great laugh, anyways something to think about !
One thing I do notice is that the shallower water pics are nicer coloured (without flash of course) as to be expected, but often a real pain due to the swell rocking you back and forward. A muck stick, or 2 fingers down on a rock is often necessary at the best of times, but in the shallows....well put it this way, on several occasions in Bali I was face to face with a mantis shrimp that didnt really like me that close, only for me to get pushed closer by the swell...fun!
The diffuser is only needed for the macro lens when you dont have the strobe on, as when you do, the velcro mask covers the cameras flash, but for some pictures the onboard was fine, just get yourself some Polymorph (mouldable thermo plastic), a milk carton and some elastic, and bobs your uncle, a diffuser which fits around the Inon Macro lens.
Weight wise it shouldnt be too bad, especially the little nano strobe, just remember some spare batteries and charger too !.
Either way.....ENJOY !
Moose