Please advise how to go to Lotus Hill by water from Xidi Pier in Guangzhou.

It is a long journey and you need to transfer several times.
From the Xidi Pier, you need to walk to the Cultural Park Station where you can take the subway line 6 to Ouzhuang Station and then change to line 5 to get to South Chebei Station. Then change to line 4 to reach Shiqi Station.
After that you still need to take a taxi to the hill in about 30 minutes.