Again how far under water?
Inside cab?
Over hood?
Over roof?
It matters as different systems are effected by how much of the rig sat in water.
For instances if water got into the cab but only a bit at the floor board, you likely only need to let everything dry out. If it was over the hood or cab, it is likely a total loss as the water has gone throughout all the motor, transmission and all electronics, cost of replacement of these systems is probably greater than replacement cost.
My examples are the 2 extremes just to let you understand why how high water got is important to understand.
Until this assessment is determined, I recommend doing nothing than dragging it out of the mud. DO NOT ATTEMPT TO CRANK IT! Flood waters are renown for being nasty dirty, if fluids are contaminated, you do not want that spread through the motor (oil and/or coolant), transmission, differential.