Yep, deep enough tread will work as a shitty propeller and steering will work as a shitty rudder. The idea is that it can do both, not that it's especially good at one or the other.

My dad once drove/boated/floated an old Volkswagen across a flooded road that a pickup truck couldn't cross. A big part of why it worked is that it wasn't weighed down by two tons of batteries.