Its all about how transmissions work, and how the fluid works.
Howstuffworks.com has good info on an automatic transmission.
With that said, and without going into a long book report here, basically its all about water getting INTO the transmission. Its not nessesarily about the deep water crossing itself, so much as leaks. Once a tiny bit of water gets into your tranny fluid, chances are the transmission will begin to slip so bad, that it overheats, or just slips to the point of not moving the truck period. I have seen it happen with my own eyes, more than once. Manual transmissions do not have this problem, since they are just gears that change positions with a shifter. Automatic transmissions rely on fluid movement etc. and water just plain doent work. He can very easily follow you down a trail, its just going to be more work for him. Basically, hes going to want to check EVERY seal at the transmission, before wheeling (rear main, trans output seal, dipstick, EVERYTHING at the transmission). If there are any leaks coming out, than water can get in. Also, raise the transmission vent. I believe its the trans dipstick tube. If thats the case, than NO water close to that high, or raise it. Finally, after every water crossing, check the fluid. Any signs of water, change the fluid right then and there (bring the old fluid out of the trail with you).
Good luck
