Tanker-Builder
A surfaced U-Boot and a Freighter exchanging gunfire! No one in my knowledge has ever done that as a diorama? Or two Heavies( cruisers) slugging it out, One low in the water and on her way down, But not quite at that stage yet? Or the Aircraft Carriers of either Navy, MortallyWounded and Sinking?
That last one has been covered, since it's easiert to "pose" for a diorama.
Issues of scale interfere with the other two.
With the 88 on a U-Boot you are looking at 1-3km range. At 1/700, 1km is 1.4m, around 56 inches. So, a 5 foot shelf is barely going to contain the two models. Which will need to be more than a foot deep to get a 500' frieghter broadside to a sub.
With "heavies" you are looking at 8-10 mile (call it 9-12km) separation. Five miles is a lot. At 1/1200 scale, 5 miles is 7.9 feet--94" or 2.4meters. So, our putative 8 foot shelf has a couple of three-four inch models on either end and a vast amount of nothing in the middile (remember this is presupposing a 5 mile range, right where a destroyer screen might be set in--15 miles needs 24'--7.3m).
Which leaves the workaround of forced perspective. Which is always complicated, specially for having to build the diorama to be viewed only in the one direction. The view over a 1/1200 sub to a 1/70 sinking frieghter would be odd at best.
Potentialy possible, though.