The problem is, the acid is a vital component of the flavor. Try mixing water with sugar to see what I mean...it is sweet, but it's not particualy good. To make it taste better, the best approach is to add some acid (lemon juice naturally gets you lemonade, but other edible acids would get you a similar effect.) Even fruit juice follows this basic flavor formula (most fruits are acidic, though watermelons are basic)
Watermelon is definitely not basic, it's just less acidic than most other fruits. But it's more acidic than distilled water (<7.0), therefore it's not basic.
