True, but dry-bags add weight, and weight is the enemy of the hiker. The trash bag idea isn't bad though as they can be pretty lightweight, but as you say, if your pack gets immersed it's pretty much useless. Before I bought my backpack I read a review for the Scrambler RT35 written by a kayaker, who had thrown his dry clothing and gear in one, rolled it down and buckled it shut, tied it to his kayak via a rope and simply let it drag behind him in the water, for several hours whilst he got on having fun in his kayak. He said that when he ended the trip, and opened up his RT35, everything inside was still bone dry...Now whilst I am not planning on following his lead, the fact I know a particular backpack can keep all of it's contents dry, even when fully immersed for hours, was enough for me to be convinced I should shortlist it as a possible candidate. But what finally convinced me to buy one was a video I found on youtube, where some dutch guy put a switched on laptop into his RT35, put it on and then deliberately fell backwards into a canal. He swam around for a bit before climbing back out, then he opened the RT35, pulled out the laptop and showed to the camera that it was still totally dry, still on and still fully working! So yes, I agree you can go the drybag route if you wish, but most drybags add weight, as do waterproof backpack covers, so simply buying the waterproof backpack can mean a lighter base weight and give you greater peace of mind too.