Question

why does magnesium float in water, yet it is denser than it?

Answer 1

Answer : This happens because on average you are less dense than water. That is why anything floats on anything else. It is also true that the weight of the water you displace is equal to your weight.

Answer 2

Magnesium reacts with hot water to form Magnesium Hydroxide and Hydrogen gas.

$Mg + H_{2}O → MgSO_{4} + H_{2}$

Magnesium starts floating on water , because the bubbles of the Hydrogen gas which is liberated sticks to its surface.