Water is the most important element for living beings. Human beings are 70% water; it participates in the creation of cells, and allows them to use nutrients. Hence the importance of proper hydration.
Fruits and vegetables are our best biological source for water since the plant has already filtered it, with melon and watermelon being the hydrating fruits par excellence.
Both melon and watermelon are quite appealing and good tasting, so we don't need to choose only one but rather we can alternate them as we please: juices, smoothies, slushies, or any other culinary invention.