Game concept
Fleet
The game also links it as Fleets.
From the game's encyclopedia
Armies who move into a Sea Zone will automatically Embark onto a Fleet. Embarking costs a sum of Gold, symbolizing the procurement and construction of ships.
While on a Fleet, Armies will be able to traverse water at a rapid pace, though they will cost 25% more maintenance and will start losing Supplies after 30 days at sea.
Fleets do not engage each other in Battles.
When an Army disembarks, the Fleet will disappear and the Army will become Recently Disembarked.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Fleet is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Armies who move into a Sea Zone will automatically Embark onto a Fleet." The game also links it as Fleets. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.