A 20-year-old Coast Guardsman, who served on the Presidential honor guard at President Reagan's inauguration, plunged into Rock Creek yesterday morning to rescue a chill-shocked motorist whose car had spun off Rock Creek Parkway and crashed through the ice.

Arnetta Gulledge, 35, of 419 Hamilton St. NW, said she had struggled out of the car after the accident into icy water well above the door handles but was soaked, panicked and hysterical and unable to get further than the car's trunk.

Other motorists had stopped and run to the snow-clad bank of the stream, she said, "but they were just as panicked as I was."

Help finally appeared in the person of Kenneth Parker of Clarkston, Wash., a sweatsuit-clad Coast Guardsman who was jogging.

"There was one man who was trying to edge out onto the ice to get to her, but he came back when it started to crack," Parker said yesterday. "I figured we had to get her out of that water, so I just went in."

He carried Gulledge to the bank where the onlookers wrapped her in coats until an ambulance arrived. She was X-rayed at George Washington University Hospital, treated for shock and hypothermia, and released later.

Gulledge said she was on her way from her home to her job at an electronics assembly plant in Virginia when the accident occurred about 7 a.m. She said another car swerved in front of her in traffic, forcing her to spin her blue Datsun off the road onto ice that carried it into the creek near the Parkway's Calvert Street exit.

Parker's icy mission evoked memories of Lenny Skutnik, hero of the Air Florida crash just over three years ago. But Parker said it wasn't the same thing at all. The water wasn't that deep and Gulledge wasn't badly hurt, although he didn't know that when he went in.

He said the water was chest deep at the spot and very cold. After a warm-up stop at the hospital, however, he went on to work.