There are many convenient routes to travel to Halong bay from major cities. The nearest airport to Halong bay is Noi Bai Airport, which is from 165 to 180 km, so it will take you about 3 hours and a half to transit from the airport to the port where La Casta anchors.
There is another airport nearby Hai Phong which is only 50 km away from Halong Bay, but not with daily flights.