Save yourself a lot of hassle and just prep your GoPro's directly in front of an AC unit. We drive into the interior of Bali to shoot Canyoning with GoPro's and the prep in the vehicle sees us turn the AC on full blast with the GoPro's open, filling the casing with dry air and have never had a problem. Also lived in Micronesia and Palau for seven years. Always prepped gear in an AC environment and again never had a problem.
For dome ports if you are also trying split shots in and out of water a surf photography secret is to rub a freshly cut raw potato slice on the dome. Something to do with the starch makes the water instantly run off leaving no droplets.
PS Pawel, I think you need to replace your glasses with a diving mask. If that jellyfish were any softer it would be renamed a 'lesser spotted soft jellyfish'.