Well here's the thing. They may be chilled about it and that's fine for them. However that doesn't stop you prep'ing a broad route (getting their buy in to either the circle or there & back path). Then plan options for stops along the way (don't bother sharing this latter bit upfront, it may not hold their interest!).
Then over each evening meal on the holiday, you can say "ABC is an option tomorrow or we could cruise a little further and go to XYZ - what takes your fancy?". Effectively you have a Plan A and a Plan B, the group discusses it, potentially even in some cases splitting up to follow different interests (and also get a *sanity break). It's not completely unknown/unplanned because you've narrowed down the options to just A or B.
You could also put a little rigidity in, by for instance saying you'd love to hit this lovely market in 'XYZ' on the Wednesday. It's the only day it runs, so you'd like to be moored up in walking distance of there the night before. If you do a good sell on it, they'll agree and you've got a specific target to ensure it's not too meandering & can also use that to ensure the previous night is close enough to make that practical. Ask them if anything is a 'must see' for them, and if it is, then that can be planned in with a target day depending on how far you need to travel each day.
As long as key milestones are agreed, how are they to know you have a cunning plan to link it all, and you appear chilled by being relaxed about 2 options you are confident in. If they ask about somewhere else signposted, a little white lie "it didn't sound that nice when I read up on it" can keep things on the straight and narrow.
On the flip side, the thought of a fully scheduled whistlestop tour horrifies me - I could never do one of those 'see Italy in a week' coach tours. I like to stop and appreciate what is around me. The boat to a degree will force this, as the pace is leisurely, and barring getting up earlier, there's not much you can do to change that pace. It's one of the great positives about a boat, as it forces more relaxation time.
At least you'll have a spacious cruiser. The English canal system narrowboats are much more enclosed and can easily lead to massive *friction. I recall once getting off and walking along the bank picking blackberries. Now back then I didn't like blackberries, but they seemed more appealing than staying on the boat to indulge the tension!
* Whilst cruisers will be better, I really would recommend trying to get an hour or more in each day where the couples go off separately to do something, or at least get a 'morning rota', where one couple has a lie in whilst the other couple get the boat underway. It gives some breathing space. One of my most enjoyable moments was starting the boat off on my own, on a grey day with persistent heavy drizzle, and for about an hour, just the company of a heron who would fly ahead when the boat approached. The calm was just what I needed. Not having to talk to the other 3 people helped as well!
regards
Ian