Passengers are armies that have been embarked on a fleet. Passengers move with their carrying fleet. Passengers cannot fight, but do take losses from any naval attrition that the carrying fleet takes.
Each fleet has a carrying capacity, which is the maximum unit weight of land units it can take as passengers. The capacity of warships and galleys is 1 army unit, where an army unit is 1000 infantry or cavalry, or 10 artillery. The capacity of transports is 2 army units. The carrying capacity of a fleet is the sum of the carrying capacity of all ships in the fleet.
The unit weight of each army on board the ship is computed separately. Because the unit weight computation rounds up any fractional amounts of men, this means there will be "wasted" passenger space if you load armies which don't have even 1000s of men (10s of artillery). For example, a fleet carrying 2 tiny armies of 1 man each still requires 2 full army units of space, which could otherwise carry 2000 men.
There is only one way to board a fleet. One or more fleets must be in a sea zone adjacent to the army or armies that wish to board, and the fleet or fleets must have enough total carrying capacity for each army that is ordered to board. Also, ships must not be engaged in battle. (They may be retreating.) Armies may then be ordered "into the sea zone". They are now trying to board.
Boarding, like other moves, requires a certain amount of time. The time varies depending on whether the army is currently in a port with "port access", or not. (You have access to a port only if you control it yourself.) With port access, the basic boarding time (for cavalry) is 10 days. Without port access (or in all non-port provinces), the basic boarding time is 20 days. These basic times are halved in the case of northern sea zones such as those in the north of the Baltic. Note that a port does not have to open into the sea zone containing the fleet to get the lower move time given for ports; it only has to exist in the settlement an army is boarding from.
Once armies are boarding, it does not matter whether or not the same fleet stays in the destination sea zone, or even whether a fleet is there at all. It only matters if a fleet is in the destination sea zone on the exact day the movement is scheduled to end. If a fleet is there, and it is not in combat, and if it has large enough carrying capacity, the army will board it. This happens regardless of whether the fleet is moving or even retreating from battle; armies can board moving fleets. If no fleet is there, or a naval battle is in progress in the sea zone, or no fleet has enough capacity, the army move is cancelled. If the army is in retreat, the army is lost.
If an army can board several fleets, it will prefer to board the fleet with the most empty passenger capacity decided at the time when the troops actually board. If there's a tie, the oldest fleet will be selected; that is the fleet created earliest in the game which will appear closer to the top of the save game file. Different methods of merging can cause the older fleet to become new or the newer fleet to become old depending on how the merge is carried out and whether there is a leader for one of the fleets. The leader's position in the save game will not change, otherwise the last selected fleet will absorb the other fleets in a merge. When a rectangle box is used to select fleets for merging, results are unpredictable.
Upon boarding, troops can be split among multiple fleets; this can be one way of creating tiny armies or obtaining round weights of men such as 5000 from a group of 5500. Send 1001 of the same type of men to board two fleets of one ship each and 1000 will be on one ship and 1 man will be on the other.
Because troops can board retreating ships, it is often nice to have two fleets capable of holding all your troops engaged in amphibious assualt where there is no land province for retreat, as in the case of a single-province island. The duplicate fleet can wait in a sea zone that is adjacent to the sea zone of the ships that were used to disembark the troops. If your troops are about to lose the battle on land, and there is a naval battle, you can retreat the naval force next to the island; this allows the land forces to retreat off the island. The fleet in the neighboring sea zone can arrive in time to pick up the men; if it triggers a battle, it will probably also have to retreat.
There are two ways of unloading passengers. One is to move the carrying fleet into a port, stopping in the port. When the fleet stops, all passengers it is carrying are automatically unloaded into the port settlement. Note that if you do not want to unload, but you do want to make port (to cancel attrition), you should order the fleet in and then add an order back out (via shift + right click). It is not possible to unload only some of a fleet's passengers when making port: it's all or nothing.
The second way to unload passengers is to move them ashore. To do this, the carrying fleet must stop in a sea zone next to a land province that the passengers are allowed to move into. Then you can select the fleet and click "Unload". This selects the passengers, who you may then order to move to any adjacent land province. You may reorganize and recombine armies, but if you want all to unload you'll have to abort the move and re-unload. Moving ashore does not cause movement attrition.
Note that there are bugs in this code, such that it can be really hard to select armies that are unloading. If you don't manage to select them with the mouse, cycle through your armies and fleets (PageUp and PageDown) until you find them. By doing so you can select individual armies so that each may be unloaded to a different province, or may be individually stop-moved so that it remains as a passenger.
Moving ashore takes the same amount of time as moving into the same province from land, unless there is access to a controlled port where no enemy is present. In that case the movement time is halved and rounded down (see land movement times).