Sounds like mobility is a problem, and you didn't even mention space below or trying to maneuver him in/out in a rough seaway. That dog'll need a lot of room just to lay down.

We keep a PFD on our dog when he's on deck or in the cockpit. Dogs tend to be a bit clumsy on boats and having that top-lft handle is a necessity if the dog goes overboard, but if the dog's too heavy it's of little use. Don't know how you could recover a 100# plus dog from the water back into a dink. You'd have to rig a crane and get down low enough to clip it, or have a ramp he's trained to use. May be problematic in rough conditions. We've had to recover ours twice now, both times our dog jumped out of the dinghy... Once to go after a swan and the other he jumped off to greet another dog. Our dog only weighs about 30# so it was not an issue.

They might all be safer and happier if the dog can stay ashore with family or friend while they cruise.