As well as skippering the boat, I also teach canoeing and I paddle with a teenage boy with aspergers every few weeks. Your words are very familiar to me.
We have progressed in well defined steps and we always discuss what we are going to do before we do it, so there are no sudden surprises. I have found that clear and literal instructions seem to help (none of this "when I nod my head, hit it !" type instructions).
For the first few weeks (when the weather was warmer), he enjoyed spending more time in the water than on it, but now he is paddling his own boat and loving it.
Today, we paddled a backwater on the river which involved negotiating a lot of obstructions like low hanging branches, logs in the water, gravel shallows, etc and he asked when we could do that again.
Although, there are behavioural similarities, every child is different and I am still learning.