I've done both medium speed river and lake kayaking. I love it and just went today! I've currently got a sit in which I like a lot. I used to take Harlow but she and I don't fit in the boat together anymore. I would like till start taking Rider and Tulsa separately more often.
We do take the yaks camping and do rivers up north. With a dog who has been taught on a lake and it's wearing a jacket with a knowledgeable "driver", do you think it's irresponsible dog ownership to take them on a medium pace river with small rapids?
I don't think it's irresponsible at all if your dog is comfortable with it. Hugo will be going on lowish class rivers as soon as he gets more boat training. You already said all the things I would suggest (life jacket, knowledgeable boater etc.) so as long as you eased into it I think it would be great. Our previous dog (lab mix) loved the water and road on several rivers with rapids. She knew that when we were in the boat she had to hold still and listen closely to commands. She only fell out of the boat once in her life and it was on a slower river when we were both captivated by a weird bird and the bow of my boat went right between two branches that were just under the surface. I managed to stop myself from tipping but she fell right out of the boat and promptly swam alongside the boat until my fiance paddled up to me and stabilized me so I could pull her back in.
That's another thing, if you're with someone else who can hold the side of your boat from their boat, it's surprisingly easy to pull a 40ish pound dog into the boat once you get the hand of it. She jumped out and swam alongside frequently. I knew that when she came up to the port side (I'm left handed) and swam really close she wanted to be pulled in and ride for awhile.
I probably don't even need to say this because I've seen you post stuff and your dogs seem super well behaved, but watch out for a**holes on rafts. I had a drunk guy throw a beer bottle at my dog once, exclaiming that she's the "biggest beaver he's ever seen."