No cost but must be arranged in advance as it is a B&B in high demand. However you can park your car and walk out to the lighthouse on the trail through the estuary. You can enjoy the view and even swim in the shallows adjacent to the lighthouse. There is also a place to sit right on the river where you can enjoy a basket lunch.
Do check the website for high tide schedules on the river. Anything over 4 feet makes the path impassable.
Get quick answers from Lighthouse staff and past visitors.