Going through water with atv?

I have a 2004 polaris sportsman 700 twin. They say your not supposed to go through water more than what like a foot deep. How do I see so many people on youtube going through water up to the handle bars? Then another thing is I read if you do go in water that high you have to do some type of maintenance to the atv afterward. What is this maintenance that i would have to do? Also is that box where your belt is in is that supposed to be waterproof? one last thing is when you go through the water how can that not screw up the electrical, since there are wires throught the atv that are not in something that is water proof?