Ship Rating is at the top of the different ratings categories for each ship you own. If you own a fleet, the view will be scrollable to accommodate all ships. You can find Ratings in the Manage tab - lower left on your screen. And you'll find that ratings only update when passengers are actually on board. See crowdedness section for more details. Ratings for all categories update automatically after about 30 seconds of adjusting customization with the exception of crowdedness. Default ship ratings in each category are set to 2.5 stars when you first get a new ship.
If you own more than one ship in one save slot, your company rating would be a combination of every Ship Rating in your fleet. Ratings can also fluctuate while you are sailing based on certain conditions. You can have as many as 5 stars per ship. There is 5 different categories that combined create your overall Ship Rating. And since there is a bonus available for better ratings, it might be a good idea to understand how to improve on each ratings category.
