The Light Transport is a unit in Age of Empires that carries land units across water. It is able to carry five units (every unit counts as one, be it a Villager or Heavy Catapult).
Light Transports have no attack and may need to be escorted by military ships. They are best used in groups, and with expensive units spread out, so that if one is lost, it is less likely to be the one with five Heavy Catapults or Centurions. Boats are also twice as resistant to conversion than land units.
"The earliest use of boats in war were primarily to carry men across rivers, lakes, or seas to raid and plunder. They were lightly armored but were armed with ranged weapons carried by the passengers. The most suitable boat for this purpose was built as a compromise between speed and capacity. Raiders did not want to spend long periods in boats making a crossing and needed to surprise their enemies. The boat also had to carry a reasonable number of raiders and have room for any booty to be brought back. The fastest boats of ancient times were galleys powered by sails when possible but mainly by oars. The Greek penteconter with 50 oars was a common transport for troops. In most cases, the crew of oarsmen became raiders when they reached their destination."