The humidity problem could be solved by not using the usual glass lids that come with fish tanks, and instead making up some mesh lids. You basically need to increase the ventilation dramatically if you are keeping them with a large amount of water.....as in the wild.
This has actually given me an idea...perhaps someone could design/invent a water python tank with a section that can be drained easily and have suitable ventilation as well as heat insulation. Water movement and filtration may also be necessary.