Ice Diamond Riddle SOLUTION ft. Vsauce's Michael Stevens

But couldnt the water level stay stay same or go higher if the temeprature difference between the ice and the water would be high enough so that the changed density of the water counteracts the diamond enough. Or how small  would the diamond need to be that this works? Couldnt that mean that every of the possible answers are right given the vague details of the riddle?