Lithium metal not reacting with water
The lithium-ion batteries do not contain pure metallic lithium, nor do they contain any metallic alloy of lithium. They contain various intercalation compounds which are relatively harmless by themselves.

When a charged battery is short-circuited (by throwing it into the water or otherwise), it may react quite violently, but that's another story. Any battery would do the same, more or less. It's about physics, not chemistry (I mean, chemical reactions would occur as well, but they don't explain the explosive part; they would occur during the normal operation all the same).