I’m in the process of constructing a new pack, and vetting foams for use in the hipbelt. I tested water absorption of yoga mat, generic 3/8″ thick white closed cell foam, and standard ridgerest foam by immersing 6″ by 9″ pieces of each in water for 2 hours shaking dry, then weighing. Results follow.

Yoga mat
Dry weight 1.6 oz
Wet weight 2.0 oz

White foam
Dry weight .6 oz
Wet weight .8 oz

Ridgerest foam
Dry weight .6 oz
Wet weight .8 oz

How much is just below the surface and how much is deeply soaked into the fabric is an open question, as are drying times. I do know from experience that yoga mat resists long term compression pretty well, and given that it doesn’t absorb too much I’ll be using it in the belt, with some stiffer foamboard reinforcement.