If Trewissick was modelled on a real place, one possible candidate could be Mevagissey, which lies 6 miles to the South of St Austell and has a harbour. There is a local headland to the North, which is called Penare Point (Penare sounds a bit like Kemare), and another headland to the South, called Chapel Point. However, it is just as likely that Trewissick combines features from more than one location in Cornwall.
According to the Mevagissy tourist website and the Wikipedia article, Susan Cooper used to holiday in the village. Mevagissy House was apparently the basis for the old vicarage. It is also worth noting that there is a place called Trevissick approximately 12 miles NNE of St. Austell - replace the v with a w and you have Trewissick - a simply way to disguise Mevagissy without moving too far from the place.