How do we return from Yangshuo to Guilin? By boat, or is there a bus? How much does the return cost?

You are suggested to take a bus to Guilin from Yangshuo North Bus Station. They depart from 06:45 to 20:30 with the frequency of around 20 mins. The duration is about 1.5 hours. It costs CNY 22 per person.
There is also a bullet train available, which departs at 09:03 with a duration of 51 minutes and ticket fare of CNY 26 for second class.

Many thanks Sally, information much appreciated. It sounds as if we'll need to stay the night in Yangshuo if we are to fit in the Liu San jie lightshow.

what are cheapest hotels but good are near in yang shuo after the crusie?

Yes, you need to stay at Yangshuo in order to see the fantastic show.

You may consider Imperial City Hotel at No. 9, Binjiang Road by the river. It costs CNY 100-150 per night in usual time. You may dial at 86-0773-8815116 to book a room.