You could try Balsamiq for quick mockups of a site rather than something like Visio. However, as Paulmorris mentions, this isn't for fluid mockups.
Also, as Paulmorris mentions, it is probably best to go with HTML/CSS, though this may be a bit more work. It could be worthwhile defining a set of "common" layouts in HTML/CSS and then keeping these in source control as your base starting points for mockups based on your common layouts. Designers can then access these, and use them as a base for creating new designs.
These source controlled common layouts could be used as both a base for mockups and production websites, and minor improvements or amendments to these layouts will benefit both designers and developers in future.
As an example of some great work on common formats for liquid layouts, check out;
There are demo pages below which may give you some ideas on how you could go about constructing some bare bones CSS/HTML for your common layouts.