Are we eligible for visa free transit?

We are flying into Pudong Int'l Airport in Shanghai, staying overnight in a hotel, then catching a ship the next day and sailing to Nagasaki, Japan, within 24 hours of arrival in China. This is the only entry into China we will have on this cruise, and our cruise will end in Singapore. Then we will fly back to the US. We are reading conflicting information on the requirements for free transit. Some places tell us we must arrive and leave by air, and others say we would be eligible if we arrive in China by air and leave by ship. Can you help us?