The Great Admiral is a Great Person acquired through naval combat.
"Great Admirals" provide the leadership necessary to guide powerful navies to victory on the open sea. They plan, implement, and execute the complex maneuvers necessary to maintain naval superiority, providing the coastal security that every civilization needs.
The Great Admiral is used similarly to the Great General- as a combat booster, but in sea warfare. What's different is its ability to instantly repair every naval and embarked unit in the same hex, as well as in adjacent hexes. This may be used to a crushing effect by a skilled player, or it may be wasted by an unskilled one! Note that the Admiral is consumed when using this ability.
One interesting perk of the Great Admiral is the fact that he can traverse ocean tiles, regardless of whether or not you've researched Astronomy. This is rarely useful, simply because it's incredibly difficult to earn a Great Admiral before the Renaissance Era - sea battles are just so rare in the early game, and Barbarian fights don't count towards earning Great Admiral points. However, if you do manage to get an Admiral early on, you can send him on an exploration mission and find the other continents and nations before anyone else! This gives you a strategic advantage of knowing where everything is and helps with planning early overseas expansions. Just be careful with Barbarian ships - the Admiral will be completely defenseless against them. Try to always stay in ocean tiles, where enemy Triremes and Galleys can't reach you!