Junks in Halong Bay

In March next year I should like to visit Vietnam and Halong Bay. I
heard that it is possible to hire small private junks but on the web I
only see junks for larger parties. We are only 4 people and should
like to sail 3 days ( 2 nights). Are small junks available and what is
approximately the cost?
How is the weather in that area in March? A lot of rain?

Rene Wildeman