Anywhere past the hubs is when you have to start worrying. First problems is that if water gets in the hub grease it contaminates it and causes wear. Some people change the grease ASAP but I checked mine after a deep fording and the grease was fine. But if you do it alot you should change the hub grease.

Second problem is water getting into the drivetrail components. Luckily all the vents are mounted up high or they have a valve that closes in water so they're usually ok for fording.

The real limit is the bottom of the doors, you can close all your windows, turn the ventalation on full fresh air to create a postitive cabin pressure but it won't keep the water out if the water is very far past the bottom of the doors.

Plus there's a whole mess of problems when the water is up to the body. The Xterra will actually start to float when the water gets too high. It won't float enough to float over the water but it will be enough to cause your tires to lose traction on the river bottom. And it the water is moving very fast it can wash you donwstream.

To sum it all up:
Water below the hubs is ok, no problems.
Water between the hubs and body you need to be cautious.
Water up the side of the doors is a cause for concern.

And in any situation don't go too fast through the water, you should go fast enough to create a small bow wave in front of the Xterra. You shouldn't be going fast enough to get the water to wash up over the hood.

Whew! That ended up being a tiny essay.