MARINE Pfc. Bradley L. Johnson, 21, shot himself in the head two weeks ago while serving aboard an assault ship that President Carter had ordered to the Indian Ocean, the corps confirmed yesterday.

Johnson died from his wounds in what the corps listed as an apparent suicide. His is the first death for the Marines since they were sent to the Indian Ocean to show the flag.

Six Navy pilots have been killed in crashes in Indian Ocean operations since the Iranian crisis began in November.

Johnson died March 28 aboard the LST assault ship San Bernardino while guarding the ship's weapons room. His body is still at sea aboard a larger ship, the USS White Plains, a combat cargo vessel. The body will be flown home via the island of Diego Garcia to the Philippines to California and then to Iowa.

The Johnson family of Exira, Iowa, was finding it impossible yesterday to accept either the fact or the manner of the young men's death. They rejected the official explanation of apparent suicide.

"We'll never belive he did it on his own," said Becky Akers, his sister. "I just got his last letter today. He was talking about coming home; helping around the farm, and going trapping this winter."

Exira is a small farming community -- a population of 966 in the 1970 census. The Johnsons live on a typical Exira farm of about 200 acres, mostly planted in corn and oats.

Bradley, his sister Becky recalled, decided to do something different than farming for a while after high school. He followed his brother, Brian 24, into the Marine Corps.

"He went in on his own," Akers said of Bradley."He kind of looked forward to following in his brother's footsteps. He only had until the first of November until he would have gotten out. It's not something we can accept."

Military operations anywhere are dangerous. The Marine Corps did not issue an announcement of Johnson's death because, as a spokesman said, such deaths fall into the "routine" category.