There is no boat from Yangshuo to Guilin. However, you could take a boat from Yangshuo to Xingping.

EXTRA: There were many (quite costly) tourist boats travelling DOWN from Guilin to Yangshuo,but the Li river current is quite strong & shallow in parts-most (all?)boats seemed to go back up river empty. Yangshuo is a top town for Chinese to visit by the way,as most consider it heaven on earth. It certainly IS something VERY special indeed.