If I come from South Gate of the Summer Palace. can I reach Long corridor with boat? and how much the fare?

Yes, you can still take the boat, but you need to walk for a while before you reach a dock. You need to walk to reach Bronze Ox where you can find the boat for Stone Old Man Pavilion and then walk to reach Long Corridor.
The ferry costs about 60 to 100RMB per person for one hour. The ferry need to be rented on your own.