If your daylilies were in a shallow section of the pond, it could be that their crowns were down in the water. That would probably cause them to rot, especially given that your climate is even warmer than mine. (I'm in N. CA, you are in S. CA.) My 1 gal, #1, and tree pots in my plastic boxes all have their crowns well above the water level.
One thing to be aware of is that even in our climate, potted daylilies can be subject to rot. I've learned the hard way that daylilies in those black 1 gal pots in the full sun, if kept too moist and/or having too much fertilizer, can rot. (Alternately, if dry, they may simply bake.) During the summer and/or when temps are high, I keep the daylilies in the smaller or darker pots in at least part shade. Daylilies in larger pots, 3-5 gal, seem to do better... as do those in pots that are not black. (If you buy perennials or shrubs and get them from the nursery in white or light colored pots,
save those pots. I've collected a fair number of white and tan 1 gal pots, and pink 1.5 gal pots (from carpet roses) over the years. Even green pots are better than black.)
(Speaking of pots and too much summer moisture, what is curious is that none of the daylilies in my plastic boxes, which get full sun (or almost so), have rotted. Why this is, I don't know. The pots are all black and they certainly have fertilizer (from the spring; I don't fertilize in the summer), and the seedling roots certainly are moist. It might be that the water surrounding the pots moderates the temperature somewhat.
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