You'd need to be located on a fairly large lock-free section of canal, to make it a 'worthwhile' day hire experience. Yes, there's some day boats which are located in places so that locks are unavoidable, but generally:
1. Lock free --> discount on licence
2. No locks - no delays which eat into the day
3. No locks - no training required to do locks
Once you've sussed the ideal location, then consider that there's probably already established day boat operators operating there. So you'd need to take a share of their market or do something unusual (like, underprice it, or provide a better boat) to get your market share. Then there's the fact that the boat's going to be bashed around quite a lot, and that you may end up paying out on insurance, or outside of insurance, to local boaters more regularly than you'd hope.
If it still looks like a goer, then bear in mind that every day its hired out, you're effectively 'tied up' because you're on call. And every weekend you'd be working - especially the nice ones - if you were a one-man operation. Otherwise, you're into hiring staff....and paying NI, income tax, etc etc and dealing with sickness/absence etc.
Lots to think about there if you're thinking of buying a business or starting one up.