The Air Niugini plane was on approach to the airport at the island, part of the Federated States of Micronesia, when it apparently fell short of the runway.
Videos posted to social media show rescue boats attending the scene, helping passengers and crew get off the half-submerged wreckage to safety.
In a brief statement, the airline said it could confirm that "all on board were able to safely evacuate the aircraft".
The accident happened in the tiny Pacific island nation of Micronesia.
There was also a 2013 Lion Air flight that overshot the runway and ended up in water when attempting to land at Denpasar, with all on board rescued by small boats.
Passenger Bill Jaynes, editor of the Pohnpei-based Kaselehlie Press newspaper said he did not even realise there had been an accident until he saw water gushing into the fuselage.
The aircraft was being operated by Air Niugini, the national carrier of Papua New Guinea.
It wasn't immediately clear whether the Boeing 737-800 plane crashed in the lagoon, located near an airport, during its take-off or landing, the outlet reported.
The plane was reportedly carrying 36 passengers and 11 crew.
An Air Niugini plane has sunk after ending up in a lagoon off Weno airport in Chuuk in the Federated States of Micronesia.
"We don't really know what happened. people were rescued by boats".
The typical passengers would be business people from Micronesia, Papua New Guinea and Australia, Harris said, as well as some tourists. Water was waist-deep in the cabin before rescuers arrived, he said.