It all depends on the kinds of images you shooting and what you're doing with them afterwords. The RX100 has incredible resolution and handles most lens needs with wet lens. Every SLR owner pretty mach hates having to choose lens before diving, I know I do. SLR's are great systems, but if you travel a lot you'll feel like a sherpa dragging the stuff around. I just bought a used Nauticam NEX7 set-up for this very reason. However, I'm still struggling to let go of my T3i system because of the lack of glass for NEX. I shoot mostly super macro abstractions and want to print very large prints, so I need the resolution. Small prints and video screen presentation simply don't need such high res.
The T3i/T34 systems are the smallest of the bunch of APS-C sensor SLR systems. I ended up going with the NA-NEX7 to build a APS-C HR system for the future, since it'll be at least 6 months before Zeiss releases their new macro 50mm, and then, I'll probably have to rob a bank to pay for it. Your flash might work fine with the optical ports because taping a sensor to the body of your G10 may not have been very efficient depending on how you attached it. I have a G10 too (great camera and incredible macro with it's zoom close-up focus feature. I wanted you to know there's a cheap Velcro system that glues to the body in front of the flash that you can get very cheap ($12 if I remember right). It'll connect 2 optical lines. I bought mine at Ocean Optical Sales in WA. (great knowledgeable guys).
The best part of the RX100 is it works with many wet lens just as it is, no need for special wide angle ports and such. I'm still debating whether I should have gone that route, but I want the highest quality macro lens I can find, along with large sensor resolution. The RX100 will do you fine and it's so small and light, considerably smaller than the NEX7. I'd go for it if I were you and add the other lens as you can afford them or they appear used. I bet your flash will work with it.
Happy diving.