Goke, Body Snatcher from Hell

The pilot receives a radio message about a bomb threat against the plane. Co-pilot Sugisaka checks the passengers' bags for the bomb, which are cleared apart from one man who had no bag. Stewardess Kuzumi opens an unaccompanied suitcase under a bench, finding a rifle. The man pulls a gun on Sugisaka and orders the pilot to fly to Okinawa. He shoots out the plane's transistor radio just as it was breaking the news about a UFO over Japan with Japanese and US Air Force fighters in pursuit. A luminous streaks past overhead, knocking out the airplane's control and causing an engine fire to erupt. The airplane crashes on an uncharted deserted isle.
Only a handful of people survive the crash: Sugisaka; Kuzumi; Mrs. Neal, an American widow; Senator Mano of the Constitutional Democratic Party; weapons exporter Tokiyasu and his wife Noriko; psychiatrist Momotake; space biologist Professor Sagai; and the young man who called in the bomb threat. The hijacker suddenly sits up, grabs Kuzumi and escapes into the jungle, encountering the spaceship. Kuzumi hides, but the hijacker steps into a clearing. A dark blob oozes towards the hijacker, whose forehead is split wide open, causing Kuzumi to scream and pass out.
Sugisaka finds the unconscious Kizumi and carries her back to the plane. Dr. Momotake later hypnotizes her to recount the events in the jungle. The teenager who called in the bomb threat attacks Dr Momotake, who falls off the cliff into the hijacker who then kills him by draining his blood.
As the survivors discuss finding water, a knock at the door is heard. Sugisaka opens it to find the hijacker lying on the ground with a big scar on his forehead. The survivors carry the hijacker inside and dress his wound. Tokiyasu then uses the rifle to force everyone out of the plane and locks himself safely inside, with the hijacker. Right after, Tokiyasu's screams are heard and the door swings open. Everyone rushes inside to find Tokiyasu dead, drained of all blood. The hijacker appears and carries Noriko off to the spaceship. At sunrise, Noriko is seen standing on a ridge. She speaks, but with the voice of the alien, the Gokemidoro. It is revealed that the Gokemidoro has invaded the earth, intending to eradicate the human race. Noriko then plunges off the ridge, shriveling into a cadaver.
The passengers argue about whether extraterrestrials would invade the earth. Professor Sagai theorizes that the hijacker was turned into a vampire. Mano challenges them to prove there are vampires, causing the others to plan to sacrifice someone to the Goke. The survivors shove the teenager outside as the hijacker slowly advances towards him.. The teenager pulls out the bomb he's been hiding and threatens to blow up the plane unless they let him back in. They do not and the teenager triggers the bomb, killing himself and blowing a large opening in the airplane, wounding Professor Sagai. Mano runs off with Mrs. Neal. When the hijacker catches up with them, Mano pushes Mrs. Neal to the hijacker to save himself. Neal shoots several times but misses. The hijacker kills her.
Mano escapes back to the plane with the hijacker behind. The remaining survivors leaves the plane to help Mano, but he runs past them, locking the plane door behind him. While Mano watches from inside the plane, Sugisaka tosses a bucket of airplane fuel at the hijacker, then sets him on fire.
The Gokemidoro crawls out of the burning hijacker, creeps in the plane and enters Professor Sagai's forehead. Sagai drains Mano, then turns to Sugisaka and Kuzumi, who escapes. Sagai follows until he is swept off a hill by a landslide. Sugisaka and Kuzumi keeps running while Sagai goes back to the spaceship. Once there, the Gokemidoro crawls out, reducing Saiga to dust.
Sugisaka and Kuzumi reaches a highway, finding every human in the cars and the city dead. The Gokemidoro informs them that no one will be spared.
In the epilogue, Sugisaka and Kuzumi are wandering on rocky terrain. In orbit around Earth a whole fleet of Gokemidoro spaceships awaits.

Question: What are the survivors trying to find?
Answer: A source for water.