I've used dry bags extensively on boats and while hiking. They are water resistant as long as you don't over stuff it and allow enough room to fold the sealing lip at least 3 times. They are generally not water proof for submersion but you don't really need this. A backpack style bag is easier to carry if you need to climb over railings and edges to get in and out of the boat. If you don't have a lot of items to protect consider staffing a smaller dry bag inside your regular bag or get a water resistant duffel such as this one:
https://www.rei.com/product/155771/p...ole-duffel-55l
I did say total immersion protection is probably not necessary but I was getting off a ferry once where large suitcases were being unloaded with a small crane and cargo net. The crew accidentally dropped one person's softsided suitcase into the water which was not fully secured by the net. Unbelievably, it floated at the surface and was retrievable. I'm guessing not everything inside was soaked because the bag was still floating. It would probably not stay dry inside if there was a constant rain though.