most of the claimed waterproof daypacks are of the vynil roll down top variety - I have one (cheap) I bought from Sierra Trading Post - bought it thinking I'd be using it on local day trips and overnighters, but I haven't really used it.
One issue wtih that type is typically no hipbelt, so not the most comfortable carry if heavily packed.

hate to give up the utilty of the "bookbag" with multiple pockets - so I just stick with the one I have and use a trash bag liner - nothing in there that water would really hurt anyways.

to me, for Quetico trips, a "daypack" is too small - I prefer the smaller back pack style (ultralight day and a half type)which has a comfortable hipbelt, a top flap pocket for stuff I want handy, and outside mesh pocket where I can restuff the raincoat if it stops raining - features not to be found in the typical waterproof daypack.  for solo trips, it is my food pack as well.

at 2500 to 3000 cu in size, this isn't just for odds and ends but is one of two main "packs"(the other is a duffle bag) when I solo, or we bring two plus one big pack if my brother comes along.

waterproof - not,  so I just use trash bag liners or waterproof pack liners (the pack and liner weighs less than vynil daypack and holds more) and everything is packed in multiple layers of plastic bags anyways

even with a waterproof roll down top vynil dry bag, I always use at least trash bag liner as a backup in case the pack got holed - and the roll top can/will tend to suck in some water if left sitting in a puddle anyways.  for just rain protection, spraying some waterproofing on the bookbag will work for a while