Just a word of warning against an overly optimistic chase after magnification. Higher magnification only helps if the level of detail in your image is limited by the sensor, eg. pixel size. If the detail is not in the optical image, just increasing magnification is just throwing away field of view and loose depth of field without being able to see any extra detail. The 16Mpix mFT sensor is equivalent to a 64Mpix FF sensor, so pixels are pretty small and you need a very sharp lens to max out the sensor. The 12-50mm lens is a decent lens but there is only so much you can expect from a kit lens and with an aperture of F6.3 at 50mm I expect resolution is limited by the lens and not the sensor.
After this depressing message some good news. Diopters and extension tubes can in principle increase the optical resolution of the image (a teleconverter never does) but the biggest gain of the diopter may simply be by shooting through less water. Still my advice would be to get the right tool for the job. The 60mm macro is a very sharp lens, will give you true 1:1 with longer working distance, focus from 10cm to infinity with no need to juggle diopters, and F2.8 to improve focusing and view finder image. Yes, you do loose the ability to shoot wider-angle images on the same dive but to some, me included, that is an advantage. The 12-50mm will remain a great general purpose lens with amazing macro-like abilities build in. And as a bonus, both lenses fit the same port. Pretty sweet.