Rub each ear of corn evenly with the olive oil, making sure to coat all sides. Sprinkle the sea salt, black pepper, and garlic powder over the corn, rotating each ear as you season to ensure the seasoning adheres well. This pre-seasoning step ensures the flavors penetrate the corn during the long, slow cooking process rather than just sitting on top.
Pour the water into the slow cooker, then carefully arrange the seasoned corn in a single layer or stacked arrangement (depending on your cooker's size). Cover and cook on high for 3 to 4 hours—the corn should be tender and slightly steamed but still retain some firmness. I like to check the corn around the 2.5-hour mark because slow cookers vary, and you want to avoid overcooking which can make the kernels mushy.
Carefully remove the cooked corn from the slow cooker with tongs and transfer to a serving platter. Immediately top each ear with a pat of unsalted butter (about 1 tablespoon per ear), allowing it to melt into the warm kernels. Sprinkle the fresh chopped parsley over all the ears just before serving for a pop of fresh color and flavor.