I would recommend a kayak with more of an open cockpit. For awhile I used my wilderness systems tsunami and it could be tricky getting into the smaller cockpit. If you are using a kayak to ferry the dog to shore then absolutly an open cockpit so the dog and you can be comfortable. Get something short, wide for stability getting in and out, and light for getting on and off deck.
something like this:
or a sit on top like this: