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":
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?To caulk the wagon means to seal it with pitch or tar so that no water can get in.

