Not that I have lot's of rescue experience or anything, but from the basic physics of the situation, driving through water is deceptively dangerous. People typically have the mistaken belief that a bigger, heavier vehicle is safer when traversing water, but that's not necessarily true. A football field-sized super tanker floats just as well as a rubber raft. It's the volume of water displaced that generates the buoyancy, so the bigger the vehicle body, the more water it displaces.

People also often underestimate the power of moving water, even slowly moving water. A slow current pushing against the large exterior of a Suburban is a lot of force per square inch.

In general, I would avoid driving over flooded roads if there's any way to go around since you can't see if there are big holes or objects that could flatten your tires under the surface. But, if I were to proceed, I would not let the water reach the frame rails. Water can rise quickly--or you could drive into a depression which suddenly puts the water over the frame rails--and like I said, it really doesn't take much depth of water to generate enough buoyancy for your tires to lose traction, even in a really big SUV. At that point, if there's any sort of current, you could be pushed downstream and flipped over or sent into deeper water quite easily.

I suppose the only exception to this advice is if the interior intentionally or unintentionally filled with water. Then the vehicle wouldn't really be displacing much water and wouldn't be very buoyant. It's like that scene in the movie Dante's Peak where they're crossing the river in the Suburban and the interior fills up with water. (But they had a snorkel for the engine, too. )