"I always thought she could do it given her internal energy, her mental and physical strength, her will of iron," said Diaz Escrich, whom Nyad has called a longtime friend.
"More than the athletic feat, she wants to send a message of peace, love, friendship and happiness ... between the people of the United States and Cuba," he added.
President Barack Obama was among a flurry of public officials and celebrities who tweeted congratulations. The president's tweet read: "Never give up on your dreams." Others who tweeted included Ryan Seacrest, who wrote "We should never ever give up ... you never are too old to chase your dreams."