Navy officials say the two were passengers in a 7 ton water truck that rolled over during a Navy- Marine training exercise called "Pacific Horizon."
Flint, 21, was based in Coronado but grew up in Fort Ann, New York.
"He was just the funniest kid, he was always just happy, I don't care what the situation was, Myke made it good,"said his grandmother Pamela Pecue from her home in Fort Edward, New York.
Pecue says Flint joined the Navy because he wanted to serve his country and learn how to operated heavy equipment.
Flint chose the Navy over the other armed services because he thought it was the "safest of all of them."
Navy officials say Flint and Shirar,27, who was from Baytown, Texas, were thrown from the water truck as they were delivering drinking water to a campsite.
"I keep asking myself why him? and how did he get thrown from this vehicle, why he couldn't stay in it," said Pecue.
The driver of the truck has not been identified and was taken to a nearby hospital for injuries suffered in the crash.
The cause of the crash is still under investigation.