He was introduced to watercolor by his father, Charles Homer, who was a self-taught artist and painter. It's important to note that Homer didn't just learn watercolor painting, he developed it into an art form that is still admired today.
There isn't a specific year recorded for when he began learning, but we know that he was painting in watercolor from a very young age. He even sold watercolor illustrations of his paintings to magazines in his early career.
While Homer is best known for his oil paintings, his watercolors are a significant part of his artistic legacy and are highly prized by art collectors.