Marco is a great host. He is very responsive (does he ever sleep, one wonders), helpful and personable. His flat is lovely. It is located in a quiet residential area, only a five minute walk from the beach, and about a 20 minute leisurely walk along the beach to the old town.
The flat is clean, quiet and well furnished. If you want to cook some meals, the kitchen is very well stocked and there is a great super market a couple of minutes away for supplies. I highly recommend Marco as a fabulous host as well as his apartment as a starting point for exploring San Sebastian.