I own a 2007 7.55 and have the original on water cover which I use as a spare and obviously when left on the water but the boat is mainly out of the water. I do not know how much Cobra want for the on water cover but it used to be about £750 I think. Recently for less than this I had made by JB Covers a console cover, two seats and helm and full overall winter cover, the quality is exceptional (had 4 covers from them now on different boats) and i am sure he could make an on water cover for less than cobra want and i am sure the quality would be stunning, his attention to detail is unbeleivable.
Another point, do make sure that the cover is breathable, I used the on water cover as a winter cover last year and the boat was constantly damp inside, this is what made me get new covers for her. If you would like pictures just let me know or the details of JB if you are south coast based....