Ferries run on the logic of cargo, not passengers. Walking on last means you leave first.
Find Your Footing
The rule holds from Lake Titicaca to the Mekong delta: vehicles board first, drive to the bow, and take up most of the deck. Motorbikes fill the gaps between trucks. Foot passengers are directed — by a gesture, by shouting, by nothing at all — to whatever space remains. Sometimes that is a narrow strip of railing at the stern. Sometimes it is a roof deck with a better view than anyone who paid more.
On the crossing from Copacabana across Lake Titicaca, passengers leave the bus entirely. The bus rolls onto a flat-bottomed barge on its own; passengers take a smaller boat alongside and re-board on the other side. Nobody announces this. You watch what the Bolivians do and follow. The two craft do not always arrive simultaneously, which means standing on a dock at altitude in the wind while you wait for the barge to bring the bus across.
The Mekong crossings in Cambodia — before bridges displaced the older routes — worked the same way: a ramp lowered into the mud, a truck reversed on with great ceremony, then motos, then people on foot paying the foot-passenger rate, which was a fraction of the vehicle fare and required no negotiation. You found a spot. The river did the rest.
What foot passengers gain is disembarkation speed. When the bow ramp drops at the far shore, vehicles reverse off slowly and in sequence. Anyone on foot is already moving. This matters when there is one onward bus and it will not wait.
What foot passengers give up is shelter. The covered seating, where it exists, tends to be amidships and claims quickly. Arrive early, or arrive ready to stand in the weather. A short crossing in sun is nothing. The same crossing in rain on a Zimbabwean lake feels considerably longer.
Fares are usually collected on the boat, not at a booth. A person with a notebook or a roll of tickets will find you. Pay the foot-passenger rate; do not pay the vehicle rate unless you brought a vehicle. If the collector does not appear before the far shore, wait — they always appear.
The logic of these crossings is cargo first, people second, but that ordering only governs the loading. Once the engine runs and the shore drops away, everyone is equally aboard, equally wet if it rains, equally watching the same water. The truck driver and the foot passenger arrive at the same time. The foot passenger just leaves faster.