Depends where are you located in Porec. If you are in the south part of Porec, you can go to Zelena Laguna camping facility. They have a part of the camping reserved for naturists. Anyway, you have to pay small entry fee. If you have a bicycle (there are a lot of possibilities to rent a bicycle or a scooter in Porec), you can drive further to Funtana, and there is a naturist camping Istra. I do not know is there any fee to pay there, was only in Zelena Laguna.