I am flying into Tianjin on the 11 Sep 19 and taking a cruise from Tianjin to Japan on the 13 Sep.

You can really enjoy your time having a Singaporean passport.

When you enter China, your 15 days start to count down. In reality, you will spend just three days from September 11 to September 13.

On September 13, you will board the ship and at that moment, the count down of your days stops because you will receive an exit stamp and since that moment you are considered to have left the country and the days spent in cruise are not counted against your 15 days because you are not in China any more.

Upon your arrival back to China, you will start again from your maximum number of days, i.e. 15 days and not from the remaining 12 days. This is the rule imposed by China and for some reason Singaporeans are treated in quite a generous way. In other words, every exit from China by the Singaporean citizen, resets the clock of 15 visa free days.