The bay in daylight is busy. The bay at six in the morning, with the mist still on the water and nobody else up, is the reason people come — and that is the only part a day trip cannot give you.
You are collected in Hanoi, run out on the expressway, and aboard by noon with a seafood lunch while the boat threads the limestone. Sung Sot Cave and Ti Top Island in the afternoon, a sunset party and a cooking demonstration on the sundeck, dinner, and a cabin with a window. Tai chi at half past six, kayaks at Luon Cave, brunch while the boat comes back in, and you are in Hanoi for the evening flight.