Robbo
Great concept and great layout. I like the figure placement. The creek bed is coming along well.
If this were my diorama here are some things I would consider.
I am a fan of using the stuff in this article for water.
https://armorama.kitmaker.net//features/150. I've used it a number of times and it has worked great. It shouldn't have any adverse affect on the delicate parts of the ZuFuss (I would test it just in case couldn't hurt).
I might work on the banks a bit and get some rocks showing a bit - or some roots sticking here and there. The bottom is a bit mono tone and in my personal taste a bit on the gray side, I'd prefer browns and greens. I (as you said) would add rocks/pebbles to the bottom.
Caviate - one picture - not live - I would offer you to consider the placement of the ZuFuss. To get a 1/2 track that far across the creek he would have to be going really fast. On a night maneuver they would probably want to reduce speed. With this in mind you may want to consider moving it closer to the 'take off' bank. Things you can consider to accentuate the story - add craters for mortar shells. This would add excitement and confusion to the situation and would justify higher speeds. May be a fallen tree under the water that the ZuFuss has skidded down to get it that far. You ultimately decide and you by all mean make it to please you.
I would though consider where the guys attaching the tow cable to the 1/2 track are going to be - when the water is poured they're going to be waist deep in water. That will be a cool touch if thats what you are going to do.
The ground could use a bit more ruts from both the ZuFuss and Panzer.
I'll end on another great touch - I like the gray wall on the elevated area. I like that you added the attention to a 'non visual' item.
This is going to be great when its done. Keep us posted.