Yes, 75% is correct. In fact, if you were to put this object into water on any planet (where there was sufficient water!) it would submerge to 75%. An object that has less density than water will submerge in water until it displaces water whose weight is equal to its own weight. On the moon, or any other planet, the change in weight of the object and water will be multiplied by exactly the same fraction. That is why the formula for submergence, Archimede's law, is usually stated in terms of mass, not weight. Objects, and water, have the same mass no matter what the gravity is.