Okay, fill up a water jug, put an ice cube in it, watch now it floats, half the cube (well, okay, not half, a bit more) is submerged, some is not. That means not as much water is being displaced as it should be. Push the ice cube down to submerge it fully, watch as the water rises... get it? Watching a tiny ice cube melt in a small jug is a poor experiment as water expands when frozen, and thus it's volume decreases when melting so the change to the amount of water in the jug would be miniscule. Try a giant ice block in a swimming pool and see the difference.
And as for those species we don't know about dying, yes, we would be crying over spilt liquid we didn't know about, but if that liquid could cure a cancer, for instance, we'd be crying even harder.