Fill flash for backlit/sunset portrait

I would use manual mode and you'll need to add some FEC (flash exposure compensation). At least plus one and more than likely more. Hard to say without being there. This is always a tough call for ettl2.

Set the shutter speed for how much you want the sky to "burn in" to the exposure.
Use wider aperture to keep the focus on the babe and to keep ISO down and flash recycle times short.

meter for the background(sky water) in manual mode to be just 1/3 to 1/2 underexposed(sunny 16 rule maybe) with f11-f16 and use fill flash at +1/2 to +1. If you want a nice blurred background then you will need neutral density filters to use a bigger aperture (f1.2-f4). Try using a reflector positioned at 45degrees.

www.planetneil.com

Have a look at Stephen's lessons #2 & #3 here:

http://www.usa.canon.com/content/Speedlite/index.html

This should anwser your questions.

Cliff

While I love sunset shots, I would say that I am far from being an expert. First thing I know I am going to do is shoot Raw. That will give me at least one (if not two) more stops of dynamic range:
http://photo.net/wedding-photography-forum/00U1Y4
I don't like using anything in the "auto" mode, including the flash; it's just one more variable you don't have control over. I will use manual, start with a shutter of 1/250, and dial in the aperture I need for the background. If shooting Raw, I might even be up to half a stop over because I like bringing the brightness and exposure down rather than up. By setting my shutter to 1/250th (the sync speed of my flash), I only need to adjust the shutter slower to let more ambient in. Doing this won't affect the flash exposure on my subject (since that is determined by ISO/aperture/distance). After that, I put my flash in manual, adjust until I get the fill I want and fire away. Keep an eye on the LCD,if you start to lose a lot of background light, open up the shutter. If you need to go in and adjust the aperture or ISO, make an equal adjustment on the flash. IE- if you open the aperture by 1-stop, lower the flash power by 1-stop. You will probably need to tweek things, but with a little practice it's fairly quick. On another note, a single on-camera flash isn't ideal! At the very least, perhaps get an off shoe cord and put it on a lightstand to either side. Better yet would be to bounce the flash into a reflector... and a gold reflector would look good for sunsets!

Thanks, everyone. I really appreciate your input. I'll give it another try.

go to custom functions and enable 2nd shutter curtain sync
The exposure is taken for the background. At the end of the exposure the flash fires.
Use FEC to control power of the flash
I do this with a 270EX and it works

What Angel says. Put the camera in M, point it at the background, dial in exposure, and shoot a few frames until you're happy with background exposure. Then point the camera at your subject and control foreground exposure with FEC.

Canon flash is supposed to go into fill flash mode automatically. All you have to do is point and shoot !