Mmmm, I’d like to have some watermelon now. I wouldn’t think it would be very healthy long term but it might be ok for awhile. Peanuts have a lot of protein and watermelon has water of course and vitamin C and other good stuff. You’d get tired of it after awhile. You’d also be missing out on other vital nutrients. It’s never healthy to only consume one or two things in your diet. Your body needs a variety of healthy foods.
You can survive on ramen noodles alone if you want. A woman has for I think 30 some years. All she ate was ramen morning, noon and night. You can find her articles online, does she look good? No…. lol
You need a lot of minerals, iron, nutrition, fat, protein, all assorted vitamins etc… everything. Watermelon and peanuts don’t exactly have many of those things.
So yes it’s possible, but it’s not very healthy at all and COULD cause many problems and less years in your life.