Running a 24V motor on a 12V boat system
Lots of issues here.  Firstly, a stepper motor requires a current-controlled driver, which is effectively a buck-mode switching converter using the motor's winding inductance as the output inductor.  What happens is that while stationary, the current will be at some chosen level (max 3A/winding on this one); your selection of which defines the holding torque.  Note the coil resistance is 0.9R, which means there will be only 2.7V across each winding, hence the need for the switch-mode driver.  The rest of the voltage is expressed across the winding inductance with a duty cycle of approximately 2.7/Vin, e.g. 11% duty-cycle for a 24V supply.

If you directly connect the windings of this motor to a 24V DC supply, it will draw 24/0.9 = 26.6A => 640W per winding and rapidly decompose into a nice little puddle of smoking char and molten copper.

The reason people use high supply voltages for steppers is that dI/dt = V/L and if you want to step the motor, you need to reverse the current in one of the coils.  The faster you can reverse the current, the quicker you can step the motor.  If you don't need to step quickly, you don't need a high supply voltage.  When you're reversing the current in a winding, the full supply voltage is applied, causing the current to change rapidly to the opposite direction.  Once it has reached the appropriate peak value in the opposite sign, the controller resumes switching to limit the current.

If you use a 12V supply instead of 24V, your max stepping rate (max speed) will be approximately halved (because the current rate-of-change is halved), and the torque available at high speeds will also be reduced.

Assuming you use a proper current-controlled driver, the heat dissipated by the motor will be the same regardless of supply voltage, it is R*I^2 in the windings.  A lower supply voltage means that the switcher operates at a higher duty-cycle.  The same current will flow through the windings but twice as much current will be sourced from the power supply.  In other words, I'm sure that this motor will be just as good for your purposes on 12V as it would be on 24V... but it is not an appropriate motor.

You should use a normal series-wound or permanent-magnet DC motor instead.  No need for a big, expensive and delicate stepper controller, plus the motor will should be cheaper and definitely more efficient than a stepper.

Windscreen-wiper or car window-winding motors might be a good place to start looking for cheap options.  In general, look for a gear-motor with integrated gearbox, you'll find one with plenty of torque if you look.