Going by GM specs for a tahoe, your suburban (basically the same setup) should be able to ford through 20" of still water at 5mph. I wouldn't attempt going through fast moving water though. Like many have said here, moving water has a lot of power, a few inches of fast moving water is enough to push a vehicle sideways.

As far as 4x4 low, what it does in a suburban is multiply your torque by about 2 through a change in gearing. Therefore, if you're trying to make it up a steep slippery hill (like a boat ramp or dirt hill offroad) it will allow your vehicle to make it up the hill without putting as much strain on the engine and transmission. This should only be used on slippery surfaces and only at a crawl. The best thing to do would be to check the owners manual, which should offer an explanation of how and when to use it. It's a bit more complex in its operation than 4x4 Auto or even 4Hi mode.