How does a submarine's design features use the properties of fluids?

Fluid dynamics are essential for most major systems onboard a modern submarine. Hi-pressure hydraulics are the primary method of activating key valves in major systems, though they all have manual backups as well should the hydraulics fail or be damaged.

For power, most western nations used Pressurized Water Reactor designs, which is the most successful to date. For the US Navy alone, over 500 Reactor years of safe operation have been completed. There is no tolerance for even the slightest mistake in the Nuclear Navy.

For Oxygen generation, Electrolysis is used by a machine known as an O2 generator (or more commonly called "the bomb"). O2 is generated from water made by a water evaporator and then distilled further for use to make O2. The O2 is pumped into storage tanks, and released into the boat as required to maintain required levels.

For weapons launch, water impulse is used to eject the weapon from the tube.

As I said also, all valves onboard a boat are also manually operable, but they are also operated by hand operated hydraulics.

Modern submarines depend on fluid science - without it, they could not function.