During our visit to Puerto Plata in November 2012, we went on the "Outback Safari" excursion. The final stop of the day was a secluded beach where we could boogie board in the waves.
Our offroad vehicle left the main road between Puerto Plata and the airport and headed along a dirt track (with lots of puddles from the recent rain), through dense vegetation and sugar plantations, towards the coast.
After a slow and bumpy drive of around 20 minutes we emerged in a clearing in the rainforest. We picked up colourful foam boogie boards from the back of the truck and made our way across a long wooden bridge (made from logs...and a little bit rickety!) to find a sandy beach and some fine waves.
As well as learning to boogie board (face the shore, hold your board in front of you and jump on it at exactly the right time to be carried by the waves), we were able to indulge in free back and neck massages on the beach. I didn't bother with one, but Emma did and she found it very relaxing.
I believe the beach in question (after spotting a road sign for Bergantin and making use of Google Maps!) is Playa Boca Nueva at Bergantin. It is only a short distance from the hotel resorts at Playa Dorada, but the rough terrain that must be traversed to reach it makes it feel much more remote and secluded than it actually is!