We've spent a couple of weeks in Unawatuna over the past 2 years and always stay at Primrose Guesthouse. It's a small, laidback place (8 rooms) with a friendly owner who can help with any issues, transport, tours etc. if needs be.
The rooms are all clean, with 4 poster beds with good mosquito nets (very much needed in unawatuna!) separate private bathroom with hot water, and an outdoor balcony/veranda area with seating. The better rooms (top floor) also have aircon and fridges.
There is wifi, but as with virtually everywhere in Sri Lanka it's only as good as the network providers, so can be intermittent.
The place has some slight sea views (if you're standing in the right place!) and is a 30 second walk from the beach and a load of restaurants and shops. Very much recommended.