Charlie,
It would help us if you could post a link to your product in question, or a image of it, so that we would could see the problems you are describing. Having Googled the name for an image, I can now be familiar with it.
I wouldn't worry about the drum being too small, it doesn't necessarily have to be a 55 gallon fuel drum. Lots of other products, such as grease, industrial chemicals, food stuffs, etc., come in various sizes and shapes of steel drums. I wouldn't worry about the sand bag size too much either, who knows where they could have come from? Maybe they are leftover flour bags from some field kitchen, used as sandbags, or whatever was found laying around? Not worth worrying about.
As for your rubber boat.....Humm. The muddy water depicted in the foreground isn't really meant to represent a shoreline, IMHO. More likely it represents the muddy conditions so common to the Eastern Front, as the boardwalks are there to keep one from having to trudge through the mud. You certainly could use this base as depicting a shoreline of a river or lake, with the tunnel leading into a underground bunker, but do you have enough room on the base to fit the rubber boat without it being too cramped? I don't know....
Hey, it's your model, do what you think looks good!