It's fine, as long as your baby is old enough for solids, over 4 months. Remember to have it appropriately puréed, if feeding on a spoon. For finger food, make sure it's small enough not to choke on. And as always, introduce only one new food, every 3-5days.
The amount of watermelon you eat determines how much potassium you get. In about half a cup of watermelon, there is roughly 85 mg of potassium.
Other sources of potassium: http://www.Drugs.Com/cg/potassium-content-of-foods-list.Html.
Rockmelon is another name for cantaloupe in australia and new zealand. Unless you have some other melon in mind, the answer is yes, cantaloupe (or rockmelon) is a rich source of antioxidants.