How thick are they? If it's not too bad, you might be able to drum sand them flat (maybe use a friends or a shop's if you don't have one). Also, perhaps you could sandwich them between to flat surfaces and very gently put slight pressure on them over several days with a little steam or moisture.
Instead of putting water on the wood, I've had very good results with setting warped panels on a lawn. With the sun on one side and the humidity from the grass on the other side it should come flat again. I'll often put cherry outside to 'suntan' before I finish it. If it's a loose panel I'll be sure to have it stickered well and then flip it regularly. Good luck.
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