There are multiple ways to enter Port Clyde. From the south, you can follow the route of the Monhegan ferry and come in between Allen and Burnt Islands following the buoys carefully; then go straight up the middle between Hupper Island to port and Marshall Point to starboard, minding the red nuns and green cans.
From the east coming from Muscle Ridge Channel or Tenants Harbor, just after passing Mosquito Island to starboard, turn to starboard (to the NW), heading in towards Marshall Point. Then curving around Marshall Point (to starboard) with its iconic lighthouse, this will bring you into the Port Clyde channel with Hupper Island on your port side.