About 2 miles of beach in the resort community of Makena reopened at noon after lifeguards and firefighters surveying the ocean found no sign of sharks in the area, Maui County officials said.
The woman, who was about 20 years old, was snorkeling at Palauea Beach when the attack occurred shortly before 5 p.m. Wednesday. The water was choppy and visibility was limited at the time.
Bystanders on shore heard the woman scream, put her on a kayak and brought her to land, said Lee Mainaga, fire services chief at the Maui Fire Department. Her right arm was severed below the shoulder, he said. The limb wasn't recovered.
The woman was taken to Maui Memorial Medical Center in critical condition. A spokeswoman there said Thursday that the hospital had no further information to release.
It wasn't clear what type of shark bit the woman. Witnesses interviewed didn't see the animal, said Department of Land and Natural Resources spokeswoman Deborah Ward.
"We will try to speak to the victim when she is cleared to speak with us," Ward said.
There have been six shark attacks in Hawaii this year through the end of July, including three on Maui, according to a state database. There were 11 shark attacks in the state in 2012.
The last time anyone in Hawaii died from a shark attack was in 2004, when a tiger shark bit Willis McInnis in the leg while he was surfing 100 yards off Maui. McInnis suffered severe blood loss and died on the shore despite rescue efforts by beachgoers, police and paramedics. The last fatal attack before that was in 1992.
State officials say fatal attacks in Hawaii are unusual considering how many people are in the state's waters.
Tiger sharks are the species most often blamed for attacks, but it's not known why they sometimes bite humans. They may be trying to figure out whether a person could be prey.
To protect against attacks, authorities recommend swimming, snorkeling and surfing with other people. They also say people should avoid the water at dawn and dusk, as this is when some shark species move inshore to eat.