People don't realize what a boat is. It's literally a hole in the water. All it takes is to displace enough water in an amount equal the weight of the object to float it.
And for fording, it takes less displacement than that for the force of water to shove something. For example, a two-ton vehicle needs to displace two tons (479 gallons) of water to float. Interior space, trunk space, trapped air, tires, spare tire, etc. can easily add up to that much. But how much does it take to simply lose adhesion to the surface? Let's say for a two ton passenger car all it needs is to reduce the adhesion by half the weight of the vehicle. So all it needs is to displace half of 479 gallons, or 239.5 gallons. If only a quarter, that would be 119.25 gallons. Now you're looking at large fish tank size, which is easily doable considering tires and passenger compartment. The force of the water pushing sideways on the tires and body and the amount of adhesion loss can be calculated, but I don't find a ready formula for it.