No. You can decide to take it really easy and stay on board as you choose on nearly all of the boats. On many of the bigger boats, it is normal to have quite a few passengers who do not cycle at all, the perfect solution for couples who are not both smitten by the cycling bug!
There are a couple of smaller boats where the normal practice is to be passenger free during the day with non cyclists visiting places of interest by minibus. Please ask us for details.