It's a gamble. If possible, arrange your trip to Ha Long another day.
If you are doing a day trip it is very unlikely you will get back in time. As suggested in reply #1, go another day or do an overnight trip (which generally get back to Hanoi around 4 or 5pm)Edited: 03 August 2018, 16:58
Thanks for the advice.
You could also consider an overnight thru Lan Ha Bay which has a shorter transfer time of 2-2.5 hours. We have a 6pm flight after our cruise. We chose to get a private car from the port to the airport to also save even more time.
Cruises end up normally around 13:00 and you are back in Hnaoi around 18:00, you have to be at the airport 2 hours before (but 1 hour is OK), which is minimum 7:00 for you, and, with the new bridge and superhighway, it takes about 40 mn for Hanoi Center-Airport, so, I do not see why it is 'unlikely". Check out with your cruise operator what time you are back at your hotel.
I would say that you are out of luck if taking a day trip. I would not want to be in your shoes