Boats are never really "an investment", they are generally something you spend on because you love the life; that said, I have certainly made money on boats which I have improved, or simply through buying and selling at the right time. I have also lost money.
As others have said, levels on canals tend to be more stable in times of heavy rain (like the unprecedented wet season we have had on the Thames for the last six months), but properly moored and attended boats are usually fine. That can be in a marina attached to a rising pontoon, attached to sturdy posts with the ability to rise and fall with the water levels, or simply moored correctly for the conditions and attended when conditions change.
On rivers you can use wider, more "boaty" boats (or sturdy sea-going vessels), on the canals you have thousands more miles of exploring. I am Thames based and have a boaty boat - I know most of the river pretty well so exploring is pretty limited for me, but I still enjoy just being aboard either on my own or with friends. It's about a harmonious environment as much as seeing new places. I converted a camper-van a few years ago - that is terrific for seeing places...