Taking user477343's comment and making it an answer (because I thought this word too):
You could be:
I start with the letter "S".
I have something to do with missiles
Strategic Arms Limitation Talks (SALT) were about limiting the number of nuclear missiles through a treaty.
Most water in the world is saltwater.
I am commonly used on submarines
Not sure... In the dining area?
and production/power plants.
There are power plants based on saltwater heating. There is something called a molten salt power plant.
I am some sort of storage.
Salt packing of meat (and other products) is a storage method.
I have 4 letters.
Salt has 4 letters