I would not site my duck house right on the edge of the canal. That would attract mink and rats as well as other predators. Get your ducks used to your land, ie their new home boundaries, for several weeks before you dream of letting them loose on the canal. They should come home from a swim either when you put out grain for them, or when it's bedtime. You would need to shut them in at night anyway. You are bound to lose some, but that happens with free range birds even when they can't just swim away.
My experience is with muscoveys and geese, not ordinary ducks, but my birds don't spend all that long swimming (in a pond) but go in a few times a day for a dip, then come back onto land. For the first couple of days you let them on the canal, make sure it's a weekend so you can watch them. If you are still unsure, only let them out at weekends.