In a pot, add the water, washed red beans, and 1 tablespoon of salt. Bring the mixture to a boil. Reduce heat and simmer, stirring occasionally, until the beans are tender, about an hour.
Drain beans and rinse with cool water.
Mix beans, watermelon, onion, parsley, feta and olive oil in a large salad bowl. Salt and pepper to taste. Serve cold.