Why are you giving up? You're supposed to solve the problem not let it beat you.
You divide your display horizontally figuring the depth of the water vs how much you need for the surface.
You don't have to use the same scale models unless you want to. A larger ship scale on the surface means the sub is further down, hiding. Make the height as much as you need, keep the width to a minimum.
Suspend the sub with a wire anchored behind the sub and in the wall of the case. There is no bottom of your ocean. Use a thick piece of clear plastic for the surface of your ocean. Texture it with Window Cauking. See if you can reproduce the explosion of the depth charge especially that great water spout at the surface.
Whenever you can hide whatever you use to support your kits. Anchor wires behind the kits, or from unexpected angles from the case. Don't get bogged down in details, like fish. They never stuck around for the fireworks. Thinking you could anchor the sub just below a oil slick decoy, anchor the decoy to the wall, anchor the sub to the slick. Huh? And don't forget to turn the sub away from the DD. The guy is running for his life,
twisting and turning to get away. Another opportunity to seamlessly anchor the sub into the wall. You don't even need to use the entire sub. Get busy build it and get it published so we can all see what you did.