Do you mean you want to ask the taxi driver to buy the tickets for you or you just wanna how to tell the taxi driver to the port? Most of the taxi drivers in Shanghai can't speak English in fact, so I think the betst way is to hire an English guide through a travel company in advance.

I can speak basic Chinese, I just need to know the address-- what should I tell the taxi driver? "I want to ride the boat on the river"? Haha. Tours are too bloody expensive...

You can tell the driver that you are going to Shiliupu Ferry Terminal (十六铺码头shi liu pu ma tou in Chinese pinyin) where the cruise started.

Thank you, Mr Hank :)