He improved the health of seamen through diet and hygiene.

He initiated extensive contact between Europeans and the indigenous peoples of the Pacific.

Cook as some of you might already know, was the son of the Yorkshire agricultural laborer, but what you mightn’t know is that he had little formal education. After an apprenticeship to a firm of shipowners at Whiteby, he joined, in 1755, the Royal Navy and surveyed the St. Lawrence Channel and the coasts of Newfoundland and Labrador. In 1768 Cook was then given command of the Endeavour and sailed on an expedition to chart the transit of Venus; he returned to England in 1771, having also circumnavigated the globe and explored the coasts of New Zealand, which he accurately charted for the first time, and East Australia. 
Cook next commanded an expedition to the South Pacific of two ships, the Resolution and the Adventure. On this voyage he disproved the rumor of a great southern continent, explored the Antarctic Ocean and the New Hebrides, visited New Caledonia. Cook’s introduction of the observance of strict diet and hygiene prevented scurvy, which had been the scourge of long voyages in this era of exploration. It was one of the seminal turning points in seamen’s health and welfare. His other key claim is that he was one of the first Europeans to make extensive contact with a wide variety of indigenous peoples.