Hello, we want to travel from Guilin to Xingping by boat on the Li River. Is this possible?

I do not think it is possible, because there is no such kind cruise route for the river. You can travel from Yangdi to Xingping or Guilin to Yangshuo.
If you take the ferry from Guilin to Yangshuo, your baggage can be taken on the boat and the ticket can be book at the hotel you live.
From Yangdi to Xingping, most people choose to take the bamboo raft, so that it is impossible to take the ferry.