If you get a proper enclosure and not a "tank", humidity is almost never an issue. PVC and Melamine enclosures hold both heat and humidity much better than glass (especially glass with a screen top). Also, with a proper enclosure comes proper heating, which also helps with humidity as radiant heat panels and various under sources of heat don't dry out the air, unlike bulbs and ceramic heat emitters.
If, for some extremely wonky reason (there are people in AZ and CA who don't have humidity problems) humidity is truly an issue, then switching substrate to something more humidity friendly and or increasing the water supply size and or a light misting once a day or two are options. But again, with a proper enclosure, hitting the 50%-60% mark most Boas thrive in shouldn't be a problem.
Also, since this is my first post in this thread... My vote is for the Boa. After adopting my friends red tail, they have become my favorite species overall. If the size is off-putting, then you could look into smaller Boas (you already mentioned the Rosy, which is gonna be one of my next purchases).