Jack Henderson, eight, raised more than £50,000 for the Royal Hospital for Sick Kids in Edinburgh with his charity Jack Draws Anything.
As a thank-you for treating his four-year-old brother, Noah, Jack drew pictures in return for a donation and included First Minister Alex Salmond among satisfied customers.
Now Jack, originally from Cockenzie, East Lothian, has emerged as a different kind of hero. The drama unfolded in Marin, California, where his family recently moved after father Ed got a job as a senior software engineer in San Francisco.
Jack, Noah, and brother, Toby, five, were spending time with their grandparents at a hotel swimming pool when Noah became overconfident about his swimming ability and dived in headfirst.
Mr Henderson writing on the charity website, said: "He was still in armbands but had not yet mastered swimming unaided.
"Jack heard the splash and looked up, he saw his wee brother struggling in the water, screaming and panicking. Jack managed to get under Noah and hold him up towards the surface. Jack himself was under the water and doing his best to keep his brother out of danger.
"Noah clung onto Jack's neck, hurting him, which made it harder for Jack to keep the wee man safe."
Jack is being hailed as a "hero" online for his quick actions but he said: "I didn't really think. I just knew I had to save Noah."
Jack launched his charity in 2011 after Noah was repeatedly admitted to the Sick Kids due to breathing problems, which often caused him to develop bronchiolitis and pneumonia.