How can I travel from Jiaxing (Nanhu) to the Wuzhen water town for a weekend day trip, please?

From JX Central Bus Station, there are direct buses for Wuzhen. The Central Bus Station is at the cross road of Wanguo Road and the National Highway 320.
I am not very clear where you are in Nanhu District, so it is hard to tell the exact buses. Generally speaking, the bus line K7, K19, K78, K92, K98 and K182 can take you to that bus station.