Here's a list of locations with splash parks in Massachusetts:
Bradley Palmer in Topsfield
Davis Farmland in Sterling
Manning Park in Billerica
Boston Public Pools & Spray Park
Boston Area Spray Parks
Northborough Splash Pad
Spring Brook Park in Bedford
Soule Center in Brookline
Beaver Brook Reservation in Waltham
Celluci Splash Park in Hudson
Veteran's Park in Dracut
Shedd Park in Lowell
Capron Park Zoo in Attleboro
Bradley Palmer in Topsfield
Davis Farmland in Sterling
Manning Park in Billerica
Boston Public Pools & Spray Park
Boston Area Spray Parks
Northborough Splash Pad
Spring Brook Park in Bedford
Soule Center in Brookline
Beaver Brook Reservation in Waltham
Celluci Splash Park in Hudson
Veteran's Park in Dracut
Shedd Park in Lowell
Capron Park Zoo in Attleboro