Shit mate I've given you wrong info got mixed up. Details I gave you were from Mahers Landing - Anderson's Inlet, Inverloch. As for your ?, There is no boat launching facility at Harmers Haven. The road that goes down there stops and becomes a 400m walking track down to the beach.
The closest ramp that I know is at Cape Paterson, map below.
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It's a beach launch but the sand is compact and to best avoid the rocks if you don't the area well, do it at low tide. From the bottom of the carpark you will only need to drive 30m on the beach.
And I can't remember there being access out of the mouth of the Powlett river on the other side up from Harmers Haven.