The bottom could be mud instead of hard bottom. Get stuck and you would need SCUBA gear to get out.
Also that huge hatch in the front doesn’t look all that thick to me. I doubt it would even slow down a armor piercing shell.
Its not a main battle tank. Its a personell carrier with a turret. The armor is designed to stop/deflect small arms fire.
And yes, a hard bottom of known depth is still going to be required. Back pressure on the valves would be an issue, as would water intrusion into all of the bearings.
Another problem is buoyancy. I’m surprised that this thin skinned vehicle isn’t positively buoyant. Even at that, it has to be very light on the treads when full submerged.
This type of snorkelling is nothing new, but has never really caught on. There are too many things that can go badly wrong.