Sea ice has a relatively small impact on water levels, because the volume of water displaced by an iceberg is equivalent to the volume of liquid water produced when it melts. (It's actually more complicated than that, but you get the idea.) Land ice is the real killer for sea-level change, because it's not displacing any water at all until it melts.
Edit: More specifically: If you fill a glass with tap water and ice cubes, and then wait for the ice cubes to melt, the water level will not change at all. If you use seawater instead of tap it will change a bit, because ice is fresh water, which has a different volume than seawater. If you fill a glass with tap water, and then put a funnel on top of the glass and fill it with ice cubes, the glass will overflow all over the place as the ice melts.