This Robens Pump Sack will easily inflate a lightweight trekking airbed without introducing humidity which inflating the airbed by mouth would cause.
What a bonus these are. I have never trusted manufacturers claims that Rucksacks are 100% waterproof - Yeah Right!
A pump  sack is a great bit of kit, pack all your spare clothes or anything else you want to keep dry squeeze out the air and roll down the top and secur with the fastex buckle then pack that in your rucksack.
Alternatively if you are on the river keep the air in and if capsize your kit will float. Nothing worse than your kit sinking the bottom of the river, but I suppose you'll know it's still dry down there!