Are they not? Many tanks can traverse rivers driving on bottom with special attachment to the main hatch. Leopard 2 does that.

As this sounded intriguing, here's a video I've found of this: https://www.youtube.com/watch?v=C26rJiOnKLk.

It appears the tank commander needs to guide the driver on directions, though...

It also appears that the tank is not waterproof, as you can see quite substantial leaking at around 30 seconds in.

Correct. The commentary says it's equipped with 2 water pumps to remove whatever leaks through.

I guess that's German engineering for less-than-perfect road and maintenance conditions. I like the redundancy too!

Learn something everyday, quite surprising that solution with pumps was even considered and that it actually safe for crew and tank internal systems.

That's the most practical solution. A tank is not a space capsule, sealing a multi-ton rotating turret is unrealistic.