Until you get the truck under cover I'd remove the drain plug and leave it open, that way any future water will run out and not collect inside the transmission. Tie a note to the drivers door handle that says "the trans plug is out, fill before driving" as a reminder!
If the water hasn't been in there long I'd drain it, fill it with new oil, drive it to stir it up then immediately drain it and re-fill. If yo don't know how long the water has been in there, you should remove the top cover and have a look at the gears for rust and pits.
Mike B
