Subject: Measuring pressure with no flow

I’m planning on building a submarine for my ME robot class that’s about to 
start.  I want to control its depth by opening a valve and allowing water to 
enter and air to escape.  At certain depths I want to shut the valve off and 
make the sub neutrally buoyant thus not sinking.  I imagine the pressures 
inside the sub and the water outside must be the same for it not to sink.  
How/what instruments will allow me to measure the pressure inside the sub and 
outside?  Some kind of sensor?  How do real subs do these? I can't touch it 
after I turn it on, it has to operate by itself.  I know how to program in 
basic so I plan on using the small circuit boards the class provides.  I’m sure 
there is also some equation like Pinside + weight = Poutside… any help there?  
Thanks for your time

Re: Measuring pressure with no flow