a structure to be at least partially submerged in a body of water;

an actuator mechanism with at least one crack being actuator by at least one of pedaling, treading, stepping, rotating and pulling exercises, couple to the structure and movable through an user's effort;

a pump submerged in the body of water and comprising a housing fixed to the structure and internally journaling a shaft on to which a first rotor is mounted coupled to the shaft, at a first housing portion and at least one second rotor is also mounted so as to be selectively coupled to the shaft, in a respective second portion of the housing, hydraulically separated from the first portion of the housing, said shaft being coupled to the actuator mechanism, whereby to be rotationally driven, to pump water;

a water collector coupled to the first portion of the housing of the pump, to receive part of the water pumped by the first rotor; and

a metering means comprising at least one first display fixed onto the structure and coupled to the water collector, whereby to indicate at least one of the outflow and volume of water pumped through the pump and, thus, at least one of the respective operation speed and amount of exercise being practiced, said second rotor being selectively coupled with said shaft by action of the user, such as to be rotationally driven in response to the motion of the actuator mechanism, pumping a respective flow of water admitted in the pump, back to the body of water; and a shift mechanism carried by the structure and coupled with the pump, so as to selectively allow couple and uncoupling of the second rotor relative to the shaft, by action of the user.