I was there last week and did the walk from the taxi drop off at Sai Wan Pavillion (see option 2 in the link below). The taxi from Sai Kung is apparently $80-90. We took a bus from in front of MacDonalds (see the sched. in the link below) for $15. When you get off (20min ride) you have a pretty easy mostly downhill hike to Sai Wan (a small beach with restaurants and rooms to rent) that takes 45mins. From there you hike up and over a ridge to get to Tai Long Wan that takes about 45mins. There is very little shade so on a hot day it can be brutal (It was!)
If there is a water taxi available, I'd be very interested to find out the details too!
Theres more info here http://tailongwan.org/tran_e.htm about getting there.
Of course I'm assuming you mean the Tai Long Wan in Sai Kung Country Park, as there is another one near Shek O (red minibus from Shau Kei Wan) and a third one up in the Western NT somewhere....