Backpack/Bag

Looks like LL Bean has a waterproof backpack:
http://llbean.com/llb/shop/124373?page=adventure-pro-waterproof-pack&bc=29-915-916-1098&feat=1098-GN3&csp=f&pos=11

That LLBean bag looks like it would be a pain to use as a traveling daybag—the way you'd have to dig everything out to get to what's sunk to the bottom. But any truly waterproof bag will be along those lines.

Zippers are the real weak point in "waterproof" anything. Sealed or flap-covered zippers would offer more protection with still being reasonably accessible. And that should really suffice for getting caught in the rain as a tourist.

More than a few choices at REI. I have no experience with any of them. Might try picking what's interesting and then Googling reviews about it.

https://www.rei.com/c/backpacks/f/f-waterproof?sort=min-price

PacSafe products are highly regarded for their anti-theft features.

How about a rain cover? Storm flaps covering zippers really help a lot. (Narrow, Fabric flap that simply lays over the zipper.). I got caught in heavy rain with a backpack. The main compartment had a storm flap. The contents remained dry. The contents in an unprotected zipper pocket were wet.

RedOxx makes heavy duty bags with a high quality water resistant zipper. My husband got caught in a deluge of rain and the contents of his RedOxx bag remained dry with only a couple of things slightly damp on their edges near the zipper. He was happy and surprised at how well his work papers were protected.

If you're concerned about heavy-duty and wear-and-tear as much as security, then consider Jandd items like this one:
https://www.jandd.com/detail.asp?PRODUCT_ID=FMB

I bought this one to use as a personal item and I absolutely love it. It's only "water resistant," so maybe it's not what you're looking for: https://www.amazon.com/dp/B003DGL6W4?ref=ppx_yo2ov_dt_b_product_details&th=1

I usually just use a non-waterproof polyester sack pack for daily outings and I live in rainy Vancouver. If I was worried about keeping something dry I could put things into a plastic grocery bag (not easy to find anymore as plastic has become prohibited in Vancouver) and throw it into the sackpack.

Thanks for the replies. While researching, I found fjallraven which I know it's the opposite direction I want to go in.

The previous posters gave good advice about waterproof being a unicorn given the zippers being the weakest link.

Any modern bag will have sufficient water repellency with a DWR coating to endure most storms you would ever want to be out in. If you enjoy hiking in downpours for hours without an umbrella, then look for a bag that has an integrated rain cover like an Osprey hiking pack.

I have a Tumi backpack that I use for city trips which has an option (that I do not have or need) for a waterproof cover. For adventure trips, I use my trusty Osprey or Lowe day hiking bags with integrated rain covers. In over 35 years of hiking, I cannot recall ever having deployed the rain cover. If it is raining that hard, I am taking cover (or staying i side the pub) as I am more concerned with getting wet myself.