In a solid (ice) the attraction between the water molecules holds them tightly packed together. Although you can't see it with your eyes, the molecules are vibrating and releasing energy as heat.
Have you ever found an old ice cube in the back of the freezer and wondered why it is smaller than a new one? The particles in the ice are moving and releasing heat, so the ice cube will get smaller over time.
Anyway, back to your question...
In a liquid, the water molecules are more loosely connected, free flowing, and release more energy as heat than in a solid.
When water boils, the water molecules are able to move even more freely and release even more energy as heat.
Think about the tremendous size of an iceberg, and how many water molecules it would take to form that huge of a solid. Now think about that pot of boiling water and how many water molecules it would take to fill that pot. (Many, many more water molecules in the iceberg, than in the pot).
Keep in mind, the water molecules are releasing energy as heat in the solid as well as in the boiling water. But because the iceberg has exponentially more water molecules than the pot of water, the total amount of heat released from the iceberg would be greater than the amount of heat released from the pot of boiling water.
:)
