This underwater vehicle includes an electrolyte-activated electrochemical fuel cell to supply it with electrical power, which fuel cell includes: an electrochemical power production cell (3), a reservoir (4) to contain the electrolyte. means of circulation (7) of the electrolyte between the electrochemical cell (3) and the reservoir (4), comprising a semi-axial flow pump arranged axially in the reservoir and comprising a motorized wheel rotatably mounted in a diffuser, characterized in that the diffuser has the general shape of a dome to direct the flow of the electrolyte coming out of the pump in a direction substantially parallel to the axis of the pump, and thus of the reservoir.