Does round trip ticket in Shenzhen Shekou port mean one has to come back same day itself?

You don’t have to be back on the same day. When you purchase the round-trip tickets, the returning date and time is fixed, it maybe one day or several days later. You have to return back right on the date.

If you want to back in advance, you need to check if the boat has vacant seat. If it has, you can take it. And the ship must be operated by the same company.

If not, you have to buy another ticket or just wait till the right time to take the ship with your previous ticket. Besides, you cannot take the ship after the fixed date and time.