The Super Moon played a role in the accident as the boat hit a rock during an unusually low tide, the fire department said in a press release.
The fire boat was used to rescue the 12 people just before the 39-foot-long boat sank after taking on too much water just after 1 p.m. Monday.
Stamford and Greenwich police departments, Stamford EMS and the Coast Guard all responded to the scene.
It was the second time in two weeks the fire department deployed the fire boat, the department said.
The group had been boating from Norwalk to Stamford to grab a meal when the accident occurred off Shippan Point.
It apparently hit rocks just outside of the East Break Wall.
Click here to follow Daily Voice Norwalk and receive free news updates.