Effective ocean-going submarines have been around for
over a century. In the beginning, they were not pleasant
places to be. Sealed in a cramped steel tube with diesel
fumes, explosives and a few tons of battery acid, the crew
were also subject to dreadful eating, sleeping and washing
facilities.
Civilized countries mitigated this privation with
ample rum/gin/schnapps/torpedo polish.
Nowadays things are different. Gone are grey-skinned
ghouls in wool sweaters impregnated with dirt, and a
month's nervous sweat. Instead, they're swanning about
with coffee machines and two pairs of shoes. Obviously,
standards have slipped and they've all gone soft. At the
expense of a little inconvenience and discomfort, I think
we can radically increase capability.
A submarine is essentially a pressure vessel turned inside
out. The latest Seawolf class of submarines use that vessel
to withstand the pressure ~3000ft underwater, which is
around 90 MPa. Interestingly, humans can adjust to ~1500ft
depth with technique known as saturation diving, where
divers bodies become saturated, or equilibrated with the
special mix of gasses they're breathing. This means they
can live and work for very long periods underwater without
repeated and time consuming pressurizing/depressurzing
cycles.
Now, in a submarine. the pressure vessel is free. Lets get
the crew in and start winding up the pressure to a steady
45 MPa with a standard trimix of He, O2 and N2. What does
that get us? Well, funny voices for one. But other benefits
present themselves.
If the submarine could dive to 3000ft before, now the same
sub can hit 4500ft.
CO2 buildup would be simple to deal with since it begins to
liquify at 5 MPa and at 45 MPa you'll have to wipe it up
with a towell.
Compressed air could be used as energy storage, with the
sub at 45MPa, an additional vessel inside could store much
higher pressures than usual, the energy density would be
triple that of lead acid batteries, but only when released
at the surface. Also, storage of O2 would be much more
efficient.
Air cooling would be vastly more efficient, the little fan on
your computer wouldn't be necessary and exotic reactor
designs could be used.
The crew would be motivated to stay aboard until the end
of the planned mission.
Everyone would feel about 12lb lighter due to boyency.
The pressure cooker won't need a lid.
At a specific depth you could open the bottom and have a
pool.
There are some downsides, the kettle would take a long
time to boil and then make an unsuitably hot cup of tea.
Any TIG welding will problematic since the argon will be
solid and your darts game will likely suffer.