FYI: I am not a survivalist. The closest I've been to water is on a beach, both an oceanic one and on a riverbank fishing one time. If you find yourself in this situation, do not rely on the methods I suggest.
You nailed two of the big ones right off the bat: dust/smoke and biological debris. But there's several other things one could use to infer the presence of dry land.
All that was stated about the locals is that they are low-tech and live on lily pads. Having not read the book (and partly from the vagueness of the terms you used), I have no idea just how low that low-tech is, but assuming they're able to swim and have been living here for some time, they're bound to find areas where the sea bed seems to slope upwards, either gradually or sharply.
If they can follow the slope upwards, then (theoretically) they should be able to at least find a sea mount, which is fairly close.
Coastal Life Forms?
If there are lily pads, then they must've come from somewhere! And if there is dry land, then there will be creatures adapted to take advantage of the halfway lifestyle. Birds, for example, cannot live forever on the wing; they need roosting places, to rest and nest. Creatures with limbs can't swim as efficiently as finned or flippered swimmers, but they can crawl on land.
So seek out life forms which are known to live at least partly on land, and follow them. They'll lead you to land.
Assuming the locals are native to the planet (I.e., evolved there), then they'll probably have stories about how they used to live on land. Such tales, whether oral or written, can be used alongside education to search for real-life places. The City of Troy, for instance, was a real city discovered by following clues left in the Illiad and the Odyssey. Such reverse-engineered maps could also lead to forgotten islands and the like.
Just some ideas to consider. Hope it helps!