Are you new to model boats as well?
Working model submarines are arguably the most difficult subject in model boating. Any drawings you get are likely to tell you what the outside looks like and what shape it is, and it should be remembered that the main purpose of most models is to make the paint (which is what you see) be the right shape and stay that way. What makes it work is up to the individual modeller, and there are a great many variations.
Generally, a model submarine will be a free flooding shell to hold the paint in place, with a watertight compartment (WTC) being the part that holds the important stuff like radio, controls, motors, batteries and probably the ballasting system.
Have a look at **LINK** scroll down the page, click on the U-boat picture and enjoy a good read. Since it was written, there have been several commercial products appear on the market to ease the problems outlined, but it is still not an easy path.