Although ice becomes water when melted in your mouth the ice does help hydrate you but you are also dehydrating yourself faster than you can hydrate from the ice.
Your body has to work harder to create heat to melt that ice in your body which causes further dehydration of the body.
Sucking on ice or eating ice is okay but it's not a good option to hydrate you especially if you're already dehydrated.
If you're dehydrated you need to drink some water and it's best to drink some water or other fluids before you actually become thirsty to stay hydrated.
Being dehydrated can lead to death if not treated in time and it gets more serious.
Our bodies need the water and other fluids to keep hydrated and keep working for our organs and kidneys to function properly.
But the ice dehydrates because of the heat being taken out of your body to melt the ice so it dehydrates you further instead of hydrating you even though ice turns to water when melted.