At dusk, white walls can lose their colour before the sky does. The harbour in the cover image is already dark at street level, while pink light remains over the roofs. A single illuminated doorway is enough to make the buildings readable.
Check how you will get back before settling down with the camera. If you depend on a ferry, confirm the current departure and boarding arrangements with the operator. If you are staying on the island, work out the route to your accommodation while you can still see it clearly.
Choose a position that leaves the quay and doorways clear. Include a strip of water and enough sky to show where the remaining light is coming from. Make a frame before the lamps switch on, then another afterwards. Keep the composition similar so the change is easy to see.
Watch the bright windows when checking exposure. They can lose all detail while the rest of the image still looks too dark. Take a second, darker frame if needed. Steady the camera, and avoid assuming that a boat will remain still during a longer exposure.
Keep the departure time in your notes with the photograph times. A caption can then explain how long you stayed and what changed. Those details turn a pleasant harbour picture into a record someone else can understand.