Kinda’ depends upon how much and where you intend to take the dog. We are limited by “gator” water. Our 83 pound, stands up after and hour and unsteadies the boat, retriever isn’t going into swamps, rivers , etc where gators frequent. But that is different than most of y’all.
In flat water we put the dog on the back of a SOT fishing kayak. The paddler gets a work out and the dog is happy to go. In the summer, we swim the dog before getting in the boat. The fishing kayak has scuppers that squirt water up when putting on some speed/power stroke. Paddler and dog is a load for a 12ft boat. If he’s wet, and cool, the squirts don’t bother him. He has his own kayak but he can’t paddle very well.
Towing the dog in “his” kayak is cumbersome. He gets tired of it all and just swims to shore. This kayak makes a better drink dispenser at parties, ice & cans.
Ocean Kayak, Tetra with dog in back.
If we were to take him regular I see a tandem with the center hatch removed making a dog cockpit. We also put him in a canoe. Lots more room in the canoe. Of course when 80 pounds of ballast gets antsy your boat trim gets disturbed. The canoe tends to be too much room to move around for ours. We did an instructional video on this which remains hidden in the files for the moment. Might post later.
There are some recreational kayaks with sit in extra large cockpits that many put the dog in also. But the lab might be too much. There are though some SOT tandems that might do the trick.