John, I''m not an expert; I'm just sharing how I would look at this problem if I were you.
The first line of defense is washing your mask daily, and your face right before you mask up. If you've spent time in a hospital, there may well be resistant staph on your skin, and therefore also on the mask. Even if the airstream is completely sterile, if it dislodges those bacteria from your face or mask, they will be blown directly into your lungs.
The second line of defense is the HEPA filter on the air outlet, because that's proven technology. HEPA blocks 99.97% of particles >0.3 micrometers dia. (Staph are about 0.6 micrometers.) Even if a few bacteria get into the tank, they won't get past the HEPA. The specs on the filters say that they shouldn't be used on an auto machine with pressure less than 7.http://www.cpap.com/productpage/In-line-outlet-filter-cpap-bipap.html
So far, we're dealing with known facts. This paragraph is speculative. I wouldn't think peroxide can get from the reservoir into your lungs. Peroxide would kill any stray bacteria that happened to get into the humidifier water, but I have no idea what strength is required, how long it would last, or whether it would react with silicone. I wouldn't rely on peroxide until I researched those questions.
If I were in your situation, immuno-compromised and going on a cruise next week, I would use good mask and skin hygiene, a HEPA outlet filter, distilled water, and disinfect the tank daily with:http://www.cpap.com/productpage/Control-III-Disinfectant-Germicide-CPAP-Mask-Cleaning-Solution-Cleaner.html
because that's a proven product, doesn't take up much space and there's time to get it before you leave.
It can't hurt to refrigerate the water, but if that's not possible I wouldn't worry, so long as I am doing the other stuff.
Just my thoughts.....
Have a good trip.