Milford Sound / Piopiotahi
Like on the postcards.
Sea kayak or other boat.
Roscos Milford Kayaks may at times provide a water-taxi service. A maritime radio can help to coordinate pick-up.
Note that a stiff sea breeze develops in the afternoon (esp on calm sunny days) which can make the sound choppy and launching difficult from places exposed to the west. This will subside at dusk, which is a nice time to paddle back without the tourist boats, but remember head-torches.