ABC News' Daniel Rapaport reports:

A dreamy Hawaiian vacation day turned nightmarish for a 20-year-old German woman when she had to be revived on the scene after losing her arm in a shark attack at Palauea beach in Maui, according to the Maui Police Department.

Local authorities were notified of the attack Wednesday afternoon after a kayaker and two of the woman's friends heard a loud scream in the water where the tourist was snorkeling.

"I heard a scream like I'd never heard before," witness Andree Conley-Kapoi told ABCNews.com. "I told everyone around me that a scream like that could only mean a shark attack."

Conley-Kapoi said a bystander from California swam out to the victim without hesitation and spearheaded the effort to get the woman to shore despite having no relation to her.

"Incredible, really incredible," Conley-Kapoi said.

Hawaiian officials arrived at Palauea, also known as White Rock Beach, shortly after the attack and found the unconscious 20-year-old covered in blood, according to police and witnesses.

Her injuries were being treated at Maui Memorial Medical Center where she remained in critical condition, a source told MauiNow.com.

ABC News reached out to Maui Memorial Medical Center, but officials there refused to comment.

A press release by Maui police described weather conditions at the beach Wednesday as choppy with low visibility. Two miles of the beach between Makena Beach Resort and Mana Kai Resort were closed immediately after the attack and will remain closed for at least 24 hours, following shark attack protocols.

This shark encounter was the second attack this week and the third of the year on the island. On Tuesday, a shark bit an abandoned kite board at Ka'a point at Kanaha Beach Park after tourist Morgan Flanner became frustrated and decided to head ashore, leaving the board behind.

"It went at it [the board] a couple times, like three or four, and it totally submerged it a couple of times," Flanner told ABC affiliate KITV. "Very thankful that it was the board and not me."

(KITV)