[The Dark]

I thought the goal with the rubber on the T-34 was to mess with thermal imaging by masking the outline of the vehicle.

For protection against HEAT, you're going to want an air gap. The goal is to detonate the warhead away from the hull and have a gap for the molten copper jet to start to disperse before hitting anything solid. The AT4 was known for being able to punch through sandbags and sand-filled crates when used as a bunkerbuster, so I don't think sand (or water) will be effective enough to justify the added weight. If you could afford the volume for it, you'd want a setup with the hull, air gap, sheet metal, air gap, hillbilly armor to maximize the number of medium changes the warhead would go through. The problem with that is now you have an awkwardly wide vehicle, so (as always), compromises need to be made between effectiveness and efficiency.