"Hydrolocking" refers to the result of a running engine sucking a large gulp of water into the intake, the "slug" of water gets past the air filter, thru the intake, and into one of the cylinders of the engine. A 4-stroke engine like we have in all of our automobiles assumes that on the compression stroke there is mostly air with a small amount of fuel vapor in the cylinder, so the piston rises in the cylinder with both intake and exhaust valves closed, so that this mixture is compressed for the impending explosion caused by the spark plug when the piston is near TDC. Well, if there is sufficient water in the cylinder (it doesn't take much), when the piston comes up, it hits water instead of air. As you probably know, water is basically incompressible, so instead of the piston compressing the contents of the combustion chamber, it slams into the water, and very very bad things happen in the engine. Usually the result is the connecting rod breaks on that piston, the piston jams into the cylinder, and the stub of the con rod slaps around in the crankcase, causing lots of collateral damage.
I have personally witnessed this happening. The engine will be running fine, and suddenly there is a loud CLANK and that's it -- you're done. It ruins your day, as it usually ruins the engine. Offroaders try to avoid this from happening by installing a snorkle device on the intake, I had one on my Land Cruiser that I just sold:
Hydrolocking has nothing to do with the exhaust.
The answer here is DO NOT drive your Lexus car in 2' of water. It's not designed for this type of situation. Even if you don't end up sucking water into the engine, you will very likely end up with water going past all the seals in the lower part of your drivetrain: transmission, differential, wheel bearings, etc. Not to mention the risk of damage to the electrical system.
Water does VERY bad things to vehicles. Don't risk it.