:. . . I think an open kayak or double paddle
: canoe will be the way to go if I want to solo paddle with him. It would
: keep the CG lower than in a normal canoe and if I make the seat location
: adjustable I can paddle with or without him.

Look at the decks on the older, large cockpit kayak designs, such as the Klepper (www.klepper.com).

Rather than start from scratch, find an existing kayak plan which meets your needs for paddler size and experience. Then alter just the deck to provide a cockpit opening (or two) which suits your needs.

If you are working with plywood, it is not too expensive to make a "test" deck from cheap lauan, screwed on to a gunwale, or chine strip. Do'nt boath glassing this, or even coatiting it with resin. Go for a few test paddles. Cut holes in it until it you get the location(s) right for your cockpit(s). Then unscrew it and use it as a template to make your "real" deck from a new piece of plywood--which you will coat with resin, and 'glass into place.

If you have a dog which likes the water it is going to be a challenge to keep the thing in the boat. Rather than fight it, let the dog out, and provide a means for it to re-enter the boat conveniently. I think something like a gang-plank would work. Consider making one about 2 to 3 feet long with one end which could hook onto the open cockpit or hatch where the dog is to go. The other end should have flotation (maybe a cubic foot or so), and the plank should be covered with an indoor/outdoor carpet or similar covering which the dog can grip with its claws.

Have fun training the dog to re-enter a boat by climbing up this gangplank!

PGJ