It appears that the Nvu project has been discontinued:
As of the release of Ubuntu 7.04 (Feisty Fawn), Nvu, a fully graphical web design program, is no longer available in the Universe repositories, since the Nvu project has been discontinued. Nvu has been revived with a new name, though--it's now called Kompozer.
Here's the bad news: because Kompozer is so spankin' new, there's no installer for it yet. You'll have to follow
these instructions to install it. It's not that hard, especially if you've worked with the terminal before. And even if you haven't, as you only have to type in some commands.

If you don't want to go through any of that stuff, perhaps you could try
Bluefish. It's not a WYSIWYG editor, which means that you'll have to learn to code HTML pages first.
Quanta Plus is another possibility. This one
is a WYSIWYG editor, but it looks a little complicated. You'll have to see what you like most.

Still haven't found what you're looking for? Here's
one more WYSIWYG tool.
Install instructionsBluefish, Quanta Plus and Amaya can be installed using the Add/Remove... tool in the Applications menu.