Welcome to the wounderful world of electric motors - the extra power may not be being delivered to the prop, and the power consumed on 12V may be less than indicated, though this is the figure off the data sheet. The extra power is being lost because you are swinging the armature of the motor twice as fast - therefore your kinietic energy is four times greater - also your frictional losses are much greater.
Whether you canuse a bigger prop on 12V depends on the torque curve of the motor - it will only work if there is enough torque to turn the prop at that speed in the water. If not all you will end up doing is disspating more heat in the system.
If I understand it correctly, there is no point in having more torque than is needed to swing a prop in water at the required speed as that extra torque is wasted. You may find that the higher voltage is less efficient when driving a bigger prop.
Wom