Tin Hau MTR to Big Buddha, Hong Kong - want to take ferry as cable car is closed when we visit.

Well, taking ferry is the not the best choice for you.
From Tin Hau Station, you can take bus no.E11 to get to Tung Chung Fire Station, Shun Tung Road first. Then transfer to bus no. 23, get off at Ngong Ping. After that, walk to the Big Buddha directly.
The duration is 115 mins and the fare is about 38 HKD per person.
And yes, it's suggested to start off earlier in order to avoid crowded people.