Definitely water hyancinth, especially if there's a lot of sun. Your turtles may develop a fondness for eating the bulbous areas. They do provide good coverage and help keep the water cooler, though (and their blossoms are beautiful).
Water lettuce has always faded away in the heat and sun for me, as has duckweed (although if it has water hyacinth to partially shade it, it's likely to grow some).
You might try some of the "---wort" plants, especially cabomba (fanwort). I put a big bunch in each of the stock tanks I had outside last summer (3), and put them in the tanks when my turtles came in for good early last fall. The bunches were ripped apart in two of the tanks, but one bunch survived surprisingly well, and is waiting to go outside again. Turtles can have different tastes.
