Agree re canoe - your back
For more reasons than the dog. If you are staying in the environments you mention, you should be able to handle the stability of a decent solo canoe. But as importantly, a lightweight solo canoe is a LOT easier to handle by yourself from car to water and back than any kayak that can handle that size dog will be. Save your back and give the dog a safer exit option, go with the canoe.
I have a dirigo 140
and love it.
great boat, stable, not too slow. Lots of room, and a very nice big open cockpit.
I could comfortably fit a dog thats 50 or so lbs if it would sit still.
I would suggest getting the 12’ model though, as most begginers have difficulty manuvering my boat.
Try a canoe
If you haven’t done so already, then give a canoe a try. I think that for what you’re looking for, a canoe is the way to go. Take a look at the Old Town Pack canoe - made of royalex, it will last you forever and it’s only 33 lbs. Fairly affordable too, as new boats go anyway. Good luck!