There is no "extra" water. Since (at normal earth temperatures) water can be in the form of a solid, a liquid, or a gas the changes in the ocean depth just indicate that the balance among the three forms has changed. Less ice and/or lower atmospheric concentration of water vapor equals more liquid water in the system and higher shorelines. Add in the (very) occasional contribution of water from a Comet strike and plate tectonics with concurrent rises and falls in land levels, and there you have it. Earth history in a nutshell.
Now if you want an explanation of why some people think that climate change is new, that's more complicated since then we'd have to discuss socialist religion and the public school system.