The situation was reported just before 6 p.m. Tuesday near the Upper Pond section of Grace Lord Park.
Authorities say they were notified that a 6-year-old child was calling for his mother and when police searched the area, they found footprints going into the water.
That is where they found a 35-year-old woman and an 11-year-old boy dead in the water.
Authorities say there is no cause for concern and it appears to be an isolated incident.
It was not clear if the 11-year-old boy was also the woman's son.
As authorities towed a Nissan SUV from the scene, local residents said drownings do happen there occasionally, but a winter-time drowning is especially shocking.
"It's obviously a very sad day when children are involved in this type of situation, horrible situation, our deepest condolences go out to the family," said acting Morris County Prosecutor Robert Carroll.
The identities of the victims have not yet been released.