You could also go to Wequassett Inn in Harwich. You can choose from the terrace in a casual setting. (Keeping in mind it IS a four star resort and you're paying a premium for it) You'll find typical New England fare there. For a super high end treat, Wequasett's 28 Atlantic is possibly the best restaurant on the Cape, Wonderful, but you need to budget $75-$100 per person exclusive of wine.
And Ocean House is a terrific rec as well.
The on the water near Harwich is Ocean House nearby in Dennisport. Local NE style fare..hmmm..well Sesuit Cafe on the other side, 20 min. from Harwich has Cape Cod fare..lobster rolls and BYOB. Also in Harwich is Brax Landing NOT TO EAT but raw bar and drinks..on the water on the deck..very pretty...
Ocean House Restaurant
425 Old Wharf Rd, Dennis Port, MA 02639
Brax Landing Restaurant
705 Main St, Harwich, MA 02645