Finding accommodations with AC in Puerto Viejo is tough. Most of the lodging is very rustic and without AC. I recall one small place in downtown PV that offered a few rooms with window unit AC (for $10 extra), but I can't remember the name.
If you don't mind driving 8 or 9 km south of PV to Punta Uva, there are two 4-star hotels, Hotel Las Palmas and Hotel Suerre, located right next to each other. They are very nice, located right on the beach
and have AC. Although not my cup of tea, I checked them out for a friend two years ago.
You can check out the websites:
Hotel Suerre:
http://www.suerre.com/PUIngles/index.htm
and
Hotel Las Palmas:
http://www.laspalmashotel.com/