Not really- too much sugar, no fiber = insulin spikes which = fat

Yes it is in my opinion it's a Fruit & it tastes good lol

yes..its mostly water

It's good just before or just after a workout. Your body needs the extra boost of sugar to perform at it's best. Plus, the way I see it... any natural food beats what you would probably be eating if you weren't eating it.

yes
it is a negative calorie food

Watermelon has lots of water (obviously) so you will get a lot of your 8 glasses a day from it. However, it has a VERY high gycemic index. It does not digest at a good pase, and it spikes your blood sugar, which makes you hungrier sooner. I believe it is the fruit with the highest glycemic index.
But, watermelon is still better for you than McDonald's and Doritos.