Hi Julian. I have to say in general I tend not to agree with the idea of buying a strobe first and the a WA lens.
The RX100 has a 28mm lens and once underwater because of the refraction it will be reflectively a 36mm.
We all know the most important rule of UW photography is to get as close to the subject as possible.
The Camera without any wide angle lens will allow you to get great results only if your subjects are small, (macro and fish portrait) anything bigger will force you to be to far away to get decent contrast and sharpness.
A wide angle lens will dramatically increase the range of subjects you can shoot maintaining the quality you expect from this little great camera.
A strobe is a great solution and is the only way to bring back the "true" colors of your subjects however is not as simple to use as plug and play. You will have to go through a steep learning curve. It will solve one big problem, color but will introduce a few more like back scatter, hot spots and exposure control and task loading.
Strobe will also have a limited range of about a couple of mt the most if you go for something like the Inon Z240 or the S&S YS-D1
I would recommend to get the lens first and practice your camera skills underwater and figure out how taking photos will affect your buoyancy. attach the lens and shoot big or small subjects alike.
Now if your main interest is the little critters, then the strobe come first but for general use the lens is a better option.