Welcome to the forum
@royal_marple and some interesting questions there.
The Recreation Ground is protected by covenants created when it was given to the community by Walter Bright Hodgkinson 111 years ago.
The covenants protect it from non-recreational use, which is why converting the toilets to a barbers wasn't allowed.
The land is lower than the canal, so it would not be viable to extend the canal into that area, even if the money and the will was there.
The possible best use for this land (in my view) would be for a new swimming pool and fitness centre. That would be recreational but I don't know if it would satisfy the covenants.
There will be a new canal side venue at Marple Wharf when the "Wharf Marple" project is finished, and there is also the Servicemen's Club next to the marina, but you are right, we do not make as much of the canal location as we perhaps could. Wharf Marple is definitely a step in the right direction.