Jacques Cousteau: When a car accident ended this French officer's aviation career, he turned to the seas with five decades of relentless exploration. In the process he helped develop SCUBA, discovered dolphin echolocation, and became the only Frenchman badass enough to be name-checked by Ol' Dirty Bastard.
Ferdinand Magellan: Magellan wasn't a quitter. Trying to sail west to the Orient, three of his Spanish captains plotted to kill him, one ship wrecked, and another deserted and went back to Spain – but he still became the first European to round South America.
James Cook: He shouldn't have been a great explorer (hell, he shouldn't have even been a sailor), but this Brit became the first European to reach Hawaii and eastern Australia, and explore much of the Pacific. But perhaps his greatest feat was convincing his sailors to eat fruits and bathe.
Zheng He: Between 1405 and 1433 this Chinese admiral led seven major voyages, including 300 large ships with tens of thousands of men, to India, Sri Lanka, Arabia, even eastern Africa. All this despite being castrated when he was 11, earning him the official title "Grand Imperial Eunuch."