Li River Cruise in March

We are visiting China in March, 2015, and are planning on taking a cruise on the Li River from Guilin. I have read that sometimes the water level is low, and boats are not able to navigate all the way to Langzhou. What is it like in March? Is March considered winter in Guilin?
Also, should tourists get malaria medication for traveling to the Li River in Guilin?
Thank you. Looking forward to this visit.
Ann Worley