If it's so wide and deep that your wagon could get completely submerged in trying to cross from one bank to the other (before terraforming), why not just put it on a raft instead? Or...
From my days of playing "Oregon Trail":
To caulk the wagon means to seal it with pitch or tar so that no water can get in.
We have pitch. We have wagons. I presume cows and horses can swim. Why not make wagons able to float if you coat the underside with pitch?