Whenever you have to cross water, the cost becomes more expensive because there are so few alternatives, ie minimal competition, so the carriers, be they by air or sea, charge what they like.
What has always amazed me about the Isle of Man is that it has never set up it's own airline (at least not since the days of Manx). With many businesses and wealthy people on the island, and with its lower tax regime than the UK, it seems odd to me that there is no established Manx-based airline. The island suffers a lot from low visibility at all times of the year, but at least if aircraft are based there, they can usually get off the island and by the time they are due to return, the visibility would usually have improved.