you can find plenty of properties in costambar listed on airbnb and similar websites. you can also check costambar monthly (available online) for more info. the beach is ok, quiet during the week and more busy on weekends. the area is pretty safe, i walk around in the evenings without any issues and security patrols plus PN plus cestur go around the neighbourhood frequently.
I live in Costambar at the top of the hill with a view of the ocean, constant breezes and few mosquitoes. You can find my condo complex online http://seahorsecondos.com/ You can negotiate for the rent. There are lots of good places for rent depending on your budget and needs. The little grocery store at the entrance has nearly everything you could want and there is a local pharmacy that can get whatever you need if they don't stock it. The restaurants offer good food at reasonable prices. The beach has a mix of toursits and Dominicans except on Sunday when Dominicans dominate. But I feel very safe here. It's not perfect. The Costambar Association of Property Owners (APC) does a good job with security but getting absentee owners to pay their dues is a challenge so our roads are not perfect. Still, I really like it here. You can always go to Sosua or Cabarete for a day or a night or two but being close to Puerto Plata is great and we're only about 1 1/4 hour from Santiago. PM me if you want more info... Judy