Alexandre Charron says his mother Andree Hamel, 70, had travelled to the Central American country to help out her niece, who was alone with two young children.
Charron says his cousin in Costa Rica told relatives in Canada the fatal accident happened in the blink of an eye while the three were at the beach on Monday.
He was told the toddler, Jasmine Rodriguez Olshansky, wandered over to the water's edge and his mother ran to get her out.
But he says the undertow was too strong and both were pulled under.
Charron says he is working to get his mother's body back to Canada so his family can start to properly mourn her death.
"It happened so suddenly," he told The Canadian Press in a French-language phone interview on Thursday.
"My mother was there to get some rest and to lend a hand."
Hamel had injured her feet in a fall years ago and needed prosthetics in her shoes, which she took off on the beach, he said, suggesting she may have lost her footing.
The family is launching a website to raise funds for the funeral and to help Jasmine's mother, he said.