On the west coast of the North Island are a number of good beaches for skinny-dipping.
Ngarunui Beach is only 5 km from Raglan, or about 50 km from Hamilton. You can walk down the hill from directly above or park near the surf club (behind the camera) and walk about 200m north to this spot.
On the eastern side of the Coromandel Peninsula this beach was saved from the developers by the government.
There is a camp ground next to the beach. You can park in the DoC parking area (just to the left of the trees) and walk about 10 minutes through the trees, then if appropriate, north on the beach.
This beach is known internationally. At the northern access you might be lucky to find a warm stream, but the usual spot for the hot water is under the sand surface (out from a rocky point) at the southern access, 1 km further on. These days, even at 'off-peak' times, this place can be crowded with textiled visitors from Asia, and since the time you can successfully dig for hot water is limited to an hour or two around low tide, there may well be better options for skinny-dipping elsewhere...