Look at the specs Jeep lists for the "trail rated" Liberty and that will give you a good idea.
In a pure emergency situation, I would not go deeper than 6-8" below the exhaust manifold as an absolute. the reason for this is if you stall, You may not restart, but at least you can get towed out with less chance of water entering the engine via the exhaust ports.
As others mention , below the hubs is quite safe, emerse the differential and you run the chance of water mixing with the grease via the vents. If you find yourself in deep water, it would be a good idea to both re-pack the wheel bearings AND change the diff fluid.
Ken