Sun won't hurt Sioux, and even if it isn't Sioux, if it has speckles on the back of the pads, it will be yellow or orange-y and the sun shouldn't bother it. I used to grow James Brydon in a mud bottomed pond...directly in the mud and it was in bloom most of the time, because it was able to stretch out and make a big clump, but in a pot it tends to bloom sporadically, but still enough to make it worthwhile. I don't recall it burning as much as some lilies, like Almost Black, which fries almost as soon as it opens here.
BTW, have you grown lilies in your mud bottomed pond before? Only reason I ask, crawfish can be a big problem in a warm climate. I see you have native fish in there, and that may keep them in check, but I had so much trouble with
them in mine. I don't like to use chemicals, and had to pump out all ponds and hand pick the crawfish each year.
Once a lily was established, they would pretty much leave it alone but every new lily that I would try to plant, they would defoliate overnight.
I don't grow lilies in ponds any more, because I can't get in and out of them like I used to. I did get in the pond a few weeks ago and got all of my koi out, and may get in one more time and plant a few lilies. I don't have as much trouble with crawfish here as I did at the old place.