"Wars are not won by evacuations." —Winston Churchill, 20th century British stateman
The Transport Hoy is the weakest "naval" unit. In fact, it is not a naval unit per se but is Rise of Kings' "flavour" of transport ship, replacing BOTH the Transport Galley and Transport Galleon from the vanilla game. Like both, the Transport Hoy is weak and can't attack in order to defend itself. Of an interesting note is that any Hoy loaded with Longbow units (namely, the Retinue Longbowmen , Yeoman Longbowmen , and Longbow Levy) tends to move faster through the water and is harder to kill, although sustained attacks from naval vessels will eventually sink them.
As in the vanilla game you must research two key technologies in order to get the Hoy: Classicism and Caravan Train from the Library , the very first two levels of the Commerce and Science technology tracks. The latter will grant access to the Dockyard, the former will then be required to allow travel by sea. This will then allow your units to convert themselves into the Hoy whenever they touch water.