We use the Simms tropical flats boots here in Hawaii and on Christmas Island. You need the ankle support when your on your feet all day. Not everywhere you walk is flat. There are uneven, rocky, and coral bottoms. Also protects you ankles from coral cuts.
Be sure to get 2 pair of the long wading socks. You pull it all the way up and then fold it down over the tops of the boot. Keeps the sand and small rocks out of your boot. (get 2, so you'll have dry soxs to put on the next morning... not as difficult as trying to put on wet ones) LOL

Also take a roll of duct tape.... Blisters and salt water are a real pain. Also some duct tape around your stripping finger is a must. Wet salty flyline pulled accross your wet unprotected finger will produce blisters in just a few hours.