Reddit reviews Concepts in Submarine Design (Cambridge Ocean Technology Series)

If you want to get a feel for what's involved in designing a range of large scale submarines and some of the fundamental principles of how they operate (which should apply to all scales of sub), get a copy of Concepts of Submarine Design. Read it cover to cover then decide if you still want to build one.

Not trying to put you off, but they're complex and dangerous beasts and should be treated with respect, they can go wrong fast. If you still want to build one after reading, fair play!

Also maybe get The Astute Haynes Manual, it goes into a surprising amount of detail about the design and build programme of a nuclear submarine.

There are probably better books and videos out there specifically relating to submersibles.