Hartley (We had a Lt Hartley i/c of the Submarine School at one time - any relation? We called him Hartley Rabbit, 1985/6).As advised 'invent' a boat using your knowledge. If you usea current class, folk will be all over your mistakes like a tramp on chips - remember Vigil on BBC recently?Causes - many. Defective weld on a bilge pump, rupture of a pipe - these would allow lots of water in. Internal disruption - a 'new experimental weapon' failure a la Kursk.
Lots and lots and LOTS of good current films on YouTube. Beware of getting US and RN terminology mixed up, eg Boomers and Bombers for SSBNs.
@WreckerL I used to love it when the Barley Sugars went out of date and got passed around!