Hello Richard,
Very inspirational photos and ideas created in your work…
Would that be the Airfix dockside crane by any chance ? If so, I expect the loco, wagon and lorry are '00 gauge' – 1/76 or similar ?
I think, so far, I may have two ways to go :-
1. Sub' + 'Mother ship' (eg Hospital /Supply etc). In this case, they would both have to be in similar scales to "look right". I'll look at the Revell U-boat as you suggest.
2. Mixed scene of railway and marine rather like yours. One idea could be the lake ferry /railway as in the WW2 film (Heroes of Telemark ?) where the wagons are actually run onto the boat – docked so as to hold the rails in alignment. More scope here I feel as scale matching not so important.
Good luck with your competition.
Dave