Eh, basically, it's because the ice cubes have a lower density than the water. Why? Because the volume of water is higher than the volume of the ice cubes (e.g. 90% of the content is water, 10% of the content is ice).
If the volumes of the water and the ice are the same, ice will definitely be of a higher density than water. Do I make sense?
There's more water than ice, so water is heavier, that's why ice floats. It's called buoyancy.
E.g. of a simple experiment I can think of...
(A) 300g of water + about 3 ice cubes
(B) 300g of water + 300g of ice
You will see in (B), that ice will actually sink.
