Perhaps a better description of the side of the house the deck is on, when you plan to 'be seated' so people better understand the issues you are trying to figure out. I am thinking that you would like to sit on the deck for lunches and/or dinners, and the sun comes in low across the yard, hence the necessity that a shading device will need to obstruct the view of the yard? Pictures are always helpful. I say an awning attached to the house that can be rolled back when not needed - you may have to place the table(s) close to the house to get the benefit of the shade, but it wouldn't need to obstruct the view...

...you may have to place the table(s) close to the house to get the benefit of the shade, but it wouldn't need to obstruct the view...

Thumbelina, is the deck you're going to build on the west side of the house?

Are you trying to eliminate that end-of-the-day-low-on-the-horizon-can't-see-anythi ng sun?

Here are two possibilities. If the deck has a roof, try a rollup-style sun shade. If no roof, try one of the privacy screens, like you'd see in an Oriental restaurant. I found a couple of websites: sun shade try http://www.backyardamerica.com/sun_shades.htm, privacy fence try http://www.nextag.com/garden-privacy-screen/search -html
Hope you find a solution, we used to live in South FL, and the afternoon sun could blind a wombat!