Water on Plata before Shabbat, but the Plata is on a Shabbat clock. If the timer only goes on Shabbat morning, for example, can I take the water off at that time when the water heats up. It seems that this is Gramma, and might be okay. If not, how does this differ from using the Shabbat timer for lights?
One can take cold or hot food off the Plata on Shabbat; it is the putting on the Plata that can be problematic. One should have the water hot on the Plata from before Shabbat and if it goes cold on Shabbat it can't be heated at all even by a preset timer. Heating and cooking differs from other Shabbat prohibitions (like turning on lights) since the sages placed a decree against leaving non-cooked food from before Shabbat into Shabbat lest one would stir the dish to increase the cooking on Shabbat. Similarly placing non cooked food or even cooked cold liquid food on a heating source on Shabbat is prohibited even if it is turned on by the timer. (Shulchan Aruch OC 253 and 254)