I have a suggestion: Add the cost of the boat into your fees and state it in the first sentence that boat transportation is included as this is the only way to access the cottage but they must make the arrangements. I would also include it in your title. " Off the Grid Cabin, Boat ccess Included" This will solve the airbnb issue and weed out the noncommitted It would need to be in the cleaning fee,and you must make it clear that is why the cleaning fee is so high.
While I live in NYC,so my hosting issues are different than yours but the common issue all hosts have is removing barriers for guests to book. By taking the issue out of their hands, I bet your booking rate will go up. I would make a bullet point of this information to the top of your description so guests must see it even though it is under Guest Access. These are major points of information in deciding to book you place, that guests need to consider. It should look something like this:
MUST READ
1. The cost of round trip Boat Access to the cabin is included in the cleaning fee - I will refund $$$ if you choose to not take the boat.
2. There is no direct car access to the property and it is 6 mile hike from the closest parking.
3. While we can provide some potatoes, kale, parsnip and herbs from our garden, you must bring your own provisions.
4. The cottage is remote so there is no cell phone service or wifi. Enjoy the peace!
5. You will be living off the grid as we create out own power & heat. Go Green!
I would then "talk up" the boat ride as an added value elsewhere. Speak about the sightseeing opportunities of the local nature and the fun of arriving/leaving by boat. Make it an intregal part of the experience of "escaping" from the modern world. You do talk about living off the grid without using that term. But you should for google & airbnb search function. This is extremely popular in the USA with shows dedicated to this lifestyle and their are many guests looking to experience this for themselves.
I truly think you have an amazing property to offer and with a tiny bit of salesmanship you can have more bookings for more money. Seriously I think the cost per night is low, especially as it seems to not change for the season. Everything that seems like a hurtle needs to be described as an asset, make that the whole point of staying with you. If you want help with your listing, let me know and I will be happy to help.
PS - I would also add more pictures - Like the boat dock, the garden, the bathroom and more scenary.