Calcium is a metal and all metals are heavier than water. So they generally drown and not float on water. Calcium is not too heavy, its relative density is only 1.5.
Calcium is a strongly reacting metal in Water. It reacts rapidly with water that it comes in touch and forms calcium hydroxide and releases hydrogen ions. This is because Calcium is more reducing by giving out electrons more readily than hydrogen and more metallic. The hydrogen bubbles that come out of water stick to the surface of Calcium. Calcium hydroxide is heavier and so it does down the water.
Due to the increased force pushing calcium surface upwards, caused by the upward pressure in the hydrogen bubbles (surface tension/internal pressure inside bubbles), Calcium stays on the surface.
But some other heavier metal may not be able to stay afloat, as their densities are higher than Calcium.