The Modern Merchant Fleet is special unit, which Caravans convert to when crossing water, similar to other land units converting to Transport Ships. It is the Industrial Age upgrade from the Merchant Fleet used in previous ages. The Modern Merchant Fleet cannot be built manually and, like all Naval Units, appears only on map types with water.
Like with all land units, Caravans cannot enter water until a nation has built a Dock and researched Commerce 1 (Barter). Furthermore, Caravans will usually only travel across water if there is a target city on a different landmass - this requires either Commerce 2 (Coinage) (to establish own cities on other landmasses) or Commerce 3 (Trade) (allows to establish trade routes to friendly/ally cities) to be researched.
Compared to the regular Caravan, the Modern Merchant Fleet is slightly slower, but has roughly 2.5x as many hit points as well as some basic armor, allowing for some protection from weak raiding attacks. Modern Merchant Fleets are stronger than any of the Transport Ships. By upgrading from the Merchant Fleet used in previous ages, the unit receives an additional +58 hit points, but no further improvements.
Despite the increase in hit points, Modern Merchant Fleets are still very weak to enemy attacks, especially from attack submarines, thus they are better off plying routes in safe waters or staying away from the sea altogether:
Caravans can be prevented from establishing trade routes across water and travelling through water by turning off the Auto Transport option using either the button or the hotkey T.