Cruise Ship Challenge: Advanced Navigation

I wear the stripes and chart the course,
Authority aboard, I'm the force.
Decisions mine when seas are rough,
Who am I when things get tough?

On oceans deep, I gauge the pace,
Not mile per hour, a seafarer’s case.
A measure old but still in use,
What’s my name—nautical truth?

I carry words from ship to shore,
Though not by voice or knocking door.
SOS and chatter I convey,
What am I that saves the day?