The important thing is to speak with the head waiter (or whatever Princess is calling them today) in the MDR. They are trained to handle dietary restrictions - waiters are not. The head waiter will see you each day at dinner and take your order for then next day's dinner and for lunch if it's a sea day. With the one day notice, they can modify a lot of foods to be low sodium.
As NavyVeteran said, just speak to the head waiter. If eating lunch in the dining room, request to see the lunch menu the night before too.
cr8tiv1 has it right too. Some are much more helpful than others.