Whilst they do seem OTT in their claims for the current state of the marina, I would have thought that the mooring terms (to which you agreed) would have clauses that state that there is no guarantee that any of the services are available at a point in time, and that you are not due any compensation in that case. They would be crazy it that were not the case. Perhaps that is challengeable given they have never been available.
For what it is worth, in my opinion with the approach that they seem to be taking I would assume that they are loosing a lot of good will, and if it were me running it and I needed the income to finish the works, I think I would be offering a discount on next years mooring. That way you get the income you need (or already have) now, and hook people in for next year when presumable you will still have spare capacity. In the meantime there is never an excuse for not being helpful and sympathetic.