You don't have to drain off the water, but canned corn will taste better if you pour it into a strainer, rinse it under fresh running water and then add water and flavorings to it as desired. For example, you might replace the can-water with broth or just add butter and the spices of your choosing before heating it up in the microwave. Classic spice choices include salt and pepper, but interesting alternatives or additions such as lemon juice or cayenne pepper can bring novelty to your dinner table.