An island is mostly rock, so if it didn't go all the way down it would sink! The exception is ice-bergs, which do float, ice being less dense than water. Look at a map of the Earth, and you'll notice that there's no land marked at the north pole. There isn't any - it's just a floating mass of ice.

Sometimes what appear to be islands rising out of the water are actually drifting masses of peat, mud, and plants. In extreme cases, these "islands," which range in size from a few feet across to hundreds of acres, can contain trees more than 50 feet tall and 8-12 inches in diameter.

An artificial island is usually constructed by dumping soil in the water until it rises above the surface. The vast majority of man made islands are constructed this way. A floating island - as the name clearly indicates - is like an anchored raft; a completely different thing.