I can't see how the proposed facility will spoil the area. The actual site to be developed is all above current drying height, so of no use to yachties. The outer areas of the Bay are a good anchorage for yachts, and would remain so.
And hardly a surprise that the normal groups are against it, they always are unless there is a slice of profit or hefty consultancy fee going their way.
Whether it's viable, well going by the number of empty berths at the new Waiheke marina, probably not.