Here are some pictures of my "rendezvous North Atlantic", a 1/350 diorama showing the encounter of U-96 and the u-boat of kptlt. Thomsen in the movie "Das Boot".  Figures are from l'Arsenal (modified) and the models are Hobby Boss.

The diorama was made using only the step-by-step description by Jim Baumann (truly excellent!) link : finewaterline The only differences are about materials :

- acrylics instead of enamel paint

- using a combination of tissue and gloss varnish or tissue and CA glue for the white wave tops - so they look transparent

- base : cut out insulation styrofoam plate (20 to 30 mm thickness- there are various thickness and qualities but I guess you can do with all of them...)

- instead of "waterlining" the model (cutting or sanding away the hull to the waterline) I keep the hull and carefully cut away the "water" (styrofoam) 

Then I  put in the painted and weathered model and fill the gaps with a product like  - "polyfilla binnenvulmiddel" (some kind of plaster) with 75% water and 25% white glue or - TEC7

I have used both and the surface does not crack yet after 3 years now...

Werner