Agreed that the Rec wouldn't be a good location. Not just because it's green space but because the topography of the land would mean a big leisure centre would stick out like a sore thumb in the middle of there. And being a "new" centre, somewhat on a periphery, it would command a load of new surfaced car parking to get through planning which would spoil the area even further.
The best place would have absolutely been the former Ridge College, already built-up, close enough to the centre of town that many people could walk. The old Peacefield Primary School could have been a reasonable spot too. Rugby club area not a bad idea but a bit out-the-way.
The only real suggestion I can offer for a new location (to probably go down like a lead balloon) would be the field in Memorial Park bordering Station Road, which I've never seen used for anything of any value. It'd be close enough to the centre of town to not require too much new parking, and would tie all the library and other facilities together in one place.
Nope, maybe just stick with the current location. It could easily fit a wider, 25 metre pool. We could even make do with the current pool, but why not build outwards at the front and onto the pointless car park at the side to provide a proper gym and new changing rooms over two storeys? Plenty of towns have swimming pools that are too small but most at least have a decent gym (New Mills and Bollington good examples). At the same time, rebuild the Derby Way car park with a deck to double its capacity.
If it's seriously going to take until January 2019 just to fix the current pool, it'll be the missed opportunity of almost 90 years if nothing else is done to improve the place. The council probably think it's poorly attended or doesn't make enough money, but that's precisely because the vast majority of Marple residents avoid it. If it was a half-decent leisure facility the patronage would easily double, triple, quadruple...