Hi,
Yes, saltwater is so corrosive that it gets gritty and the little nozzle can't rotate...a good soak in vinegar will help it move like new again. Depending on how gritty and frozen it is...you may have to soak overnight or a few days in a bucket.
Your probably right. Not enough water pressure to make it rotate. if you didn't install a ball valve on either line, it would have been easy to cut one side off to see if it would have rotated it.