I agree, something needs to be done with the whole area, but I don't think that squeezing seven houses into the site is the answer. A productive use could easily be :
Warehouse cafe/heritage centre/fitness room and outdoor area for seating and heritage items
Yard for New Horizons
Facilities for boaters'
Parking spaces for users
Open space for users of the Wharf
To be blunt, there's already attempts to convert the Wharf building in the form of The Wharf Marple Project, and according to their Facebook page they're over a third of a way there for funding.
http://www.thewharfmarple.co.uk/Let's be blunt. It's put your money where your mouth is time. If you want that facility invest and make it happen. If enough people do, then it will most likely happen.
Me? I'm skeptical that there's the demand/need for conversion to a heritage centre/cafe that needs to be satisfied. But others feel differently, so good luck to them. If they can raise the money and make it happen, that's fine by me. And if they prove me wrong, even better.
But if they don't succeed, what's going to happen to that building? I've said it before here and I'll say it again. Attempting to block redevelopment at a planning level on the basis of "it could be used for" would be simply bonkers. If the community can't raise the funds to open it up, then why would anyone else do it? In such a situation, trying to block redevelopment would simply mean the building sits there rotting away.
As for facilities for boaters? What facilities do you want? And is there a demand/need from boaters for such facilities? It's easy to say "we need this?" but I'd like evidence. I'd like data please.
Parking spaces for users? Which users? How many of them can't walk from the sizeable car parks already located in Marple? If you're talking users of New Horizons, then be specific. I can't see many people supporting general parking there myself.