How can I purchase the Li River cruise tickets in advance if I'm not staying at a hotel in Guilin?

My husband and I arrive at GL airport at 10:40 a.m. on October 6th. We are staying in Yangshuo for 3 nights. We would like to take the cruise to Yangshuo on Oct 6th. Will there be a cruise leaving GL in the early afternoon on Oct. 6th? Do we need to purchase tickets in advance. Will it be dark when we arrive in Yangshuo? How an we purchase tickets in advance if we are not staying at a hotel in Yangshuo.