Fiordland is full of fiords, but only two of them are relatively accessible from the Southern Scenic Route. One of them is Doubtful Sound, which requires a boat trip across Lake Manapouri and then a bus over Wilmot Pass to the sound.
You need to organise the trip with one of the operators as the best way to see the Sound is to go out on a boat. Options include an overnight stay, which we strongly recommend if you have the budget. The spectacular photos provided by @rogerstravelpics are from an overnight stay.