Between the three you listed, the 6" will work the best, although 8" will give slightly better corners - just a bit. Standard port will produce significant distortion underwater at wide angles, and wet lenses won't work at all with 10-18mm - it's too wide. In fact, while I don't own one, I'm not sure 10-18mm will even fit in it - it's sized for the 16-50mm, and 10-18mm is slightly longer when zoomed out. I did some testing a while ago, comparing 4", 6" and 8" domes and the standard flat port with 7.5mm fisheye, 10-18mm and 16-50mm lenses at various focal lengths and apertures; you can see the results here.