Richard:
Think of it this way. Even when floating there's a lot of (frozen) water above sea level. In one lump. When it melts, it all spreads out - a thinner layer of course, but still an extra layer of water.
Just the point I made, as it melts it shrinks until it matches the volume of the water previously displaced by the volume of ice previously below the waterline. This should not result in an increase in sea-level.