I don't know much about boat travel in Laos but here's climate information from
http://www.tripadvisor.com/Travel-g293949-...When.To.Go.htmlCool and Dry Season: November - February
Av tem Av rainfall
17 - 29 oC 26mm
From November through to early February temperatures are pleasant and rainfall is low
Throughout the year temperatures drop quite dramatically in mountain areas such as the Bolovens Plateau and Xieng Khuong Province - take adequate cold weather clothes when visiting these areas.