Based on the PET polymer, I would expect PETG to also absorb water.

PETG despite what you might have been told before is crazy susceptible to moisture in it's raw resin form and above all the filaments we make both for ourselves and for contract jobs our PETG resin has to be dried in a resin drying silo able to reach negative 40f.I was surprised when one of the largest brands of USA made 3D printer filament approached us about taking over their PETG extrusion and the first thing they told me was that, "...PETG is great to run cause you don't need to dry it..." - boy were we surprised when we started running it only to find out we had to dry the PETG resin nearly double the time of an ABS or PLA.
found in this link: http://www.makergeeks.com/pefibymafi1c.html