I also have flat narrow feet just like yours, sound like you're my Twinnie!
I found some great sandals about 3 years ago, and now have 4 pairs, they are soooo comfy, and are made to get wet too! The straps are edged with satin material so no nasty blisters and because the straps are velcro you can pull them tight enough to fit your feet, would highly recommend them.
They are called The North Face El Porto Convertible, I'm in the UK but you can get them in the States, just found them on a website by Googling:
http://www.usoutdoorstore.com/outlet/the-north-face-el-porto-convertible-womens.html
Oh yes, the back strap velcros off too, so they are also great to wear as a slide by the pool as you can slip you feet in/out of them easily without the back strap.

Don't be put off by the round nobbles on the inside sole of the shoe, as you can't feel them when wearing, that was worrying me a bit, as I have quite thin skin under my feet, but no problems at all and have walked for miles in them many times.
Hope that helps.
