My parents had canal boats for most of my childhood and we had our Labradors on them every holiday. Be aware that the canals narrow at bridges so the bank is right next to the boat, and that presents an easy opportunity for dogs to get off. Ours was known to hop off at one bridge, take himself for a wander along the towpath keeping pace with us then hop back on at the next one. Getting off canal boats is very easy, especially at the back where there is usually just a platform, so keep them at the front where there is usually more of a barrier if they're likely to leap off unbidden.
There are things to chase everywhere (ducks, people etc) so lots of things they might jump into the water after. Canals are often shallow at the sides, in many places too shallow to get the boat alongside, so you may have trouble getting them back on if they do get off. If you have a harness or similar that they're happy to wear, that makes it much easier to haul them back on board rather than just a collar.
Shut them down below in locks, unless there are enough of you to keep eyes on them at all times - locks come with all sorts of attendant dangers and also often have roads nearby.
Canal water is often dirty and minging.
I'm sure you've thought of this already but towels, towels, towels, more towels and then some towels...
Tow paths are narrow and therefore entertaining if they're in the habit of carrying sticks (yes, dearly departed Ddog, I do remember you swiping 2 very unamused people into the water with a large stick that was slightly wider than the towpath...)
I'm making it sound awful but actually it will be brilliant - new walks everywhere, lots of new people to make a fuss of them... I'm quite jealous now, my parents sold their boat years ago. Have a lovey time :)