If you haven't used a wet macro lens before then you are probably best off going for the inon ucl-165 m67.
It is a very widely used and reputable close up lens which will definitely get you into the territory of true 1:1 macro with you camera, since the g15 has quite a bit of zoom and good close focus (youll be zooming in in order to make the lens work its macro magic)
If you really want to go further later its as simple as getting another one and 'stacking' the two together for double the magnification.
Going with something more powerful to begin with will possibly be overwhelming, since it will give you extreme magnification of the subject and very slim depth of field, making the execution of an in focus and well framed shot more frustrating. It will be far easier and rewarding to start getting the hang with a less powerful lens in the beginning. In any case, the g15's long telephoto end gets more mileage magnification-wise from a lens like the inon ucl-165 m67 than other compacts that dont have that much telephoto reach, or have a larger sensor.
Here is a useful demonstration of the impact of using one, then two, 165's:
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Another lens to consider would be the subsee +5 which is considered to have great image quality.
FYI i have the same camera as you and have thoroughly enjoy using my Dyron DY.UCL67II, (which is also sold by bluewaterphoto.com as their bluewater +7 67mm macro lens)
I think it may have slightly higher magnification than the inon (which to my knowledge is closer to a +5). Ive honestly never felt the need for greater magnification yet since i can fill the entire frame with a shrimp's head - or a fish's eye - at full zoom. Also, since you often shoot at low ISO's, high shutter speed and strobes when doing macro, theres usually reasonable room to crop further in post with the g15's 12mp images since they are sharp and have low noise.
A final piece of advise to help you in your choice would be to check out the Facebook group "Underwater Macro Photographers" - if you are a Facebook user. They will add you if you join and you can search using the search bar within the group for camera and macro lens names to see what is popular and what works well. You will find tonnes of people using the inon ucl165 on there, as well as g-series cameras.
Hope this helps and you find the lens that is right for you.
