My parents, who grew up in Bergenfield in the 1950's, remember this place.
As late as the 1950's, the main road from Bergenfield to Hackensack went over the old wood-plank bridge at the Steuben House. All the kids from Bergenfield (and other towns) would congregate on Main Street in Hackensack. My understanding is that they would take a BUS to Hackensack, and often hitch-hike back to Bergenfield. This sounds fairly absurd. Nowadays, kids do neither.
Maplecrest Beach can be seen on old USGS (US Geological Survey) maps. This was not a natural feature, but a hole excavated to a level below the water table, and lined with sand. The water may have come from the ground, but to call it a a "spring" would not be accurate. But I'm sure it made for a great marketing ploy by the management.
I believe the exact location ran from Hackensack Ave to the Staples store. But check an old USGS map to be sure.