I'm on a NCL cruise arriving 4/23/2020 in Shanghai departing 4/24/2020 at 4:00pm, do I need a china

Your cruise is NCL 16-Day Ultimate Asia. I can immediately tell you that you are perfectly eligible for 24 hour visa free transit in Shanghai and you don't need any kind of visa. As you can see, your itinerary is eligible because the portions that interests Chinese authorities is:

Hong Kong - Shanghai - Hiroshima

You should not worry about the possibility that your ship is late or if the arrival is earlier than planned because it is only the scheduled times of arrival and departure that are taken into consideration. Also, you are informed that US citizens, along with 53 other citizenship are eligible for 144 hour visa free transit. So, there are no problems at all.

You will be allowed to board the ship in Singapore and you will be probably asked about your Chinese visa. You will just reply that you will visit Shanghai relying on 24 hour visa free transit. They can check whether you are eligible and they can find only one detail... That you are eligible for this visa free transit option without any doubt.