Ahh, after looking back through the history on that relation, I see where the problem was. Until I added it, that relation never had the tag "natural=water". All it had was a name tag, so of course it wouldn't display on the map. The renderers had no way to know what it was. I'm surprised no one ever realized that and fixed it. Anyway, now that it has the correct tags, it should work if you remove the lines you added closing each individual way. I know this, because it displayed properly after I edited it. For a multipolygon, the tags almost always need to be on the relation and not on the member ways.