Of all the ways in which we can prepare vegetables, boiling is perhaps the worst thing we can do to them. While it's not necessarily true that boiling vegetables reduces the efficacy of their nutrients, it is true that some of the chemical compounds responsible for their flavors tend to leak into the water as they boil. This is especially noticeable in carrots.
We've become accustomed to fruit-flavored foods tasting nothing like their fruity counterparts, from artificial banana essence to cherry-flavored things tasting more like medicine than fruit, but what specifically makes artificial watermelon taste so off-base from its authentic fruity muse? Well, it's simply because we don't yet have the capabilities to synthesize watermelon's complex flavor, which includes compounds like (Z)-3-hexenol, citrulline, and amino acids.