wireless window/door sensor

are there sensors particularly well suited to functioning and holding up to the humid environment found in bathrooms?

This depends a bit on the panel type, but in general yes. I’ve not noticed too many issues with using a standard DW10 for 2GIG panels on a bathroom window, but if you are concerned about conditions (room does not have a fan, etc.) then you might try an outdoor rated version, like the Honeywell 5816OD.