As a zoologist Hornell published a number of papers on marine organisms working most notably with his father in law Joseph Sinel in Jersey, and in 1900 traveled to Ceylon (now Sri Lanka) to report on the pearl fisheries.
After working for several more years in India, organizing the fisheries of Madras, he retired, and thus began his next career as an ethnographer of seafarming and maritime life.
He traveled extensively around the Indian Ocean world and east Asia, making records of indigenous watercraft, sailing on Junks and Sampans, and as a member of an expedition to the south seas made many records of the watercraft of Polynesia.
Hornell in the 1930s became the principal authority on traditional, indigenous watercraft particular logboats, skin boats, canoes of all types, floats and even small ships.
His work is distinguished by careful observation and measurement and supported by drawings and photographs of seafaring life all but vanished over the second half of the twentieth century.