Congratulations! We have a Bluefin with G10 (almost the same), what we don't have is the fathom 90, we have the wetmate 65. For us, we found the flat port works well in very silty water during some of our work dives we do and also for macro as the wet mate is "zoom through" but has its limitations(the 90 degree would fix this though as i understand it). As for the filter, I have tried to use it twice and it has fallen off and got wedged between the camera lens and the housing port effectively ruining the dive, the problem appears to be a lack of space. L&M says a diopter won't work because it is slightly thicker and there is not enough room. Based on what I have seen with the filter I imagine you might run into the same problem with a diopter.
One of my dive buddies has an external diopter that flips down, but he never uses it, because the camera can focus and zoom through the standard point to almost anything he would care to shoot macro wise. So he uses the standard port so he can attached his external diopter.
Either way, just check and make sure you can actuate the filter/diopter prior to diving it, obviously. I had done this but it still failed underwater. Let us know if it works, maybe i'll get one!
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