Portrait on the beach with 580EX?

I'm planning on taking an engagement photo for my sister in the next couple of days. It's going to be a casual shot, so I was thinking of just doing something at the beach with one 580EX II for fill flash, mounted on an umbrella just above me on a stand. Here's a picture of the location I was thinking of doing it at (i.e. Have the bride and groom sit on the log and use the city as backdrop). They are getting married right by that beach and she lives really close, so I thought it would be a good idea....if anyone has any better ideas for the Vancovuer area, let me know.

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I hope they are going to be close for umbrella shot using 580, background looks very light, I personally would use the 580 with the diffuser lowered and meter for center weighted. Just my opinion.

Sitting on the dirty log with a white dress, fun times...

Sorry for how my brain works