Do not shy away from an activity just for fear of litigation. With that mindset, you may never leave the house.
Anyway, for the activity:
Take the Safe Swim Defense and Safety Afloat courses
Follow them as detailed and enjoy the activity.
Put in some basic guidelines and mainly maintain discipline. If canoeing the big issue is familiarity with actually canoeing. Getting in and out is where you get injuries. Orient and train them, practice holding the paddles, etc. If swimming make sure you have the swim tests, and if in a lake, lots of kids these days do not have exp