Blue dress for beach wedding?

I'm going to a beach wedding this summer in outside Seattle. The bride wants us to be her 'something blue'. I'm 5'5", and very pear shaped. I'd like to spend under $250 on the dress. I generally don't like anything that's too floral and I hate high necklines, and I'm having trouble finding something that could work.

Anyone have recommendations for where I can look for dresses? Or ones they think might work?