Nine people were on board the plane, but there were no casualties in the incident, according to Marine Corps spokesperson 1st Lt Hailey Harms.
Lt Harms added that the conditions of the nine personnel aboard were being assessed.
It is not known what caused the crash.
The aircraft could be seen sitting upright with its wings slightly above the waterline in Kaneohe Bay off Marine Corps Base Hawaii on the main island of Oahu, north of the capital of Honolulu.
Wind gusts were up to 21 miles per hour at the time of the crash, with mist limiting visibility to around a mile, according to the National Weather Service.
The US Navy’s Third Fleet said the jet, which is used for US Navy operations and can carry both torpedoes and cruise missiles, was “on a detachment in support of maritime homeland defense” at the time of the crash.
The aircraft is normally based in Whidbey Island, Washington, a military spokesperson said, and can also be used to hunt for submarines and for intelligence gathering missions.
P-8 aircraft are also flown by the Australian, New Zealand, British, Norwegian and Indian militaries, according to Boeing.
In 2020, a US Navy P-8A aircraft was intercepted by two Russian Su-35 aircraft while flying over international waters in the Eastern Mediterranean.
Meanwhile, earlier this year, the US Navy sent a P-8A aircraft through the Taiwan Strait, prompting China to scramble its fighter jets, escalating tensions between Taiwan and China.