The bad news is there are no brakes on a boat. Thus you have two options to slow down. Firstly, just put the throttle into neutral (so the engine is not turning the propeller) and the boat will naturally slow as the resistance of the water pushes against it. Secondly, put the engine into reverse, which tries to pull the boat to a halt.
Unfortunately, neither of these are very good options. Let me explain: in the first example, when you put the engine into neutral there’s no water to speak of going past the rudder other than the 1-2mph flow of the boat drifting along the canal. No water flow means very little steering and no matter how much you waggle the tiller or turn the wheel, your boat is (more or less) not going to respond. Since you’re probably trying to slow down because you were about to hit something, a lack of steering is a bad idea at this point.
You’ve got to have a tangible amount of water coming off the propellor and past the rudder if you want to have steering control, so that means the engine must be at least in tickover (the smallest amount of power while being in gear) in order to be able to turn the boat, including when you’re trying to avoid thumping into someone.
In the other example, slamming the engine into reverse, there are two issues. The propeller isn’t really designed to work backwards (they’re intended to push water behind the boat, not drag it from ahead) so you don’t actually get that much stopping power unless you give the engine a lot of welly … but when you do that, you get a phenomenon where the turning of the prop actually spins the boat around so what happens in reality is that although you slow down, your boat ends up sideways across the canal. Getting out of this scenario can be even worse than whatever you were trying to avoid.
So, what’s the solution? Well ideally you concentrate on your steering and on what’s coming up ahead such that you can slow down gently by reducing your throttle in enough time that you can avoid whatever’s up ahead with space and time to spare. The worst thing to do is panic and start frantically revving the engine forwards or backwards, you’ll just end up losing control.
If you have been paying attention to what’s up ahead, canal travel is so slow that you nearly always have plenty of time to a) notice hazards, b) react and c) control the boat past them without a problem.
So when you spot a potential problem up ahead, the most sensible option is that you reduce the throttle to tickover; this’ll be little enough that the boat slows down due to the resistance of the water but still enough that you retain steering. Once you’ve passed whatever you needed to slow for, you can throttle up again.