The aircraft was flying from the island of Pohnpei in Micronesia to Port Moresby, stopping at Micronesia's Weno Island on the way.
Air Niugini said its aircraft had "landed short of the runway" amid reduced visibility due to rain and poor weather. An investigation is due to begin, airport officials said.
Local boaters took everyone on the flight to safety.
Airport manager Jimmy Emilio told The Guam Daily Post: "No serious injuries ... everybody is alive."
Chuuk State Hospital acting medical director Dr Siana Shapucy said that eight people were admitted to the hospital on Friday afternoon. One was on a ventilator while some of the other patients had various fractures.
A team from Auckland, New Zealand, was en route to Weno to try to salvage the plane.