For many rivers and canals, you apply online to the Canal and River Trust. Dead simple, and you can print out your day ticket on the day.
For rivers not on the list, you would need to do a Google search. For example, if I want to go on the Ouse near Huntingdon, I found I can either apply in advance, or simply go to one of two marinas that sell tickets over the counter. It's about £8.50 for a day.
I do not object to paying the day ticket, although for a small boat it is likely you would get away with it if you didn't pay. Any navigable river in this country is likely to have had work done on it: dredging to mantain the depth, weirs to maintain the water level, flood banks, moorings etc.