Depends on the condition of your door seals andhow long you are in the water.
The bow wake will push the water away from the trk, so as long as you are moving I would say three feet no problem. Measure the height of your door sills, the water will come in through the door handles and down the window seals. Max depth is the middle of the door.
If at anytime you stop moving you are toast, and NEVER back up in deep water.

 

Sounds like some snorkel envy going on.