Serious Eats has covered this :
By contrast, watermelon and pomegranate juices are thin with no body, so they need some special handling to make their textures as thick and creamy as berry or stone fruit sorbets. It's even trickier with citrus like lemon, lime, and grapefruit; not only does their juice lack pectin or fiber,** they're so tart they need extra sugar to balance their flavor, and even when you add enough, the resulting sorbet isn't as rich.
** Whole citrus fruit has plenty of pectin but it's all in the rind, not the juice or flesh.
The master ratio above works great with any fruit purée that has some body and viscosity. But what about thin juices like watermelon, pomegranate, and citrus? Without any fiber or pectin they tend to produce a thin and icy sorbet, even when made with the correct amount of sugar. What's more, they're less forgiving than berry or stone fruit sorbets, because there's nothing in them besides sugar to inhibit the growth of big ice crystals.
If you're dealing with citrus juice you have another problem: the juice is so tart it needs to be diluted and sweetened with care. Go ahead: try making lemon sorbet with four cups of lemon juice and one cup of sugar: you'll get something so lip-puckeringly sour you'll barely be able to choke it down.
The solution to both of these problems is an alternative kind of sugar, one with different sweetening and freezing properties than sucrose, a.k.a. table sugar.
Sucrose is fairly sweet and doesn't add much body to a syrup. That's why pastry chefs look to liquid sugar like invert sugar, glucose, or dextrose, which all make sorbet creamier when used properly. The easiest alternative sugar—the one you can find in any American supermarket—is plain 'ol non-high-fructose corn syrup. Trust me: it's lemon sorbet's best friend.
I've written a whole article on the benefits of corn syrup in sorbet, but here are the Cliff's Notes: 1) corn syrup is highly viscous, so it makes for richer, creamier sorbet; and 2) it's only one third as sweet as sugar, so you can use three times as much of it as sucrose—making your sorbet three times as creamy—without over-sweetening the end result. In a blind taste test, tasters almost universally preferred lemon sorbet made with corn syrup compared to sugar. You can see the difference in texture here.
Lemon sorbet made with different proportions of corn syrup to table sugar. The more corn syrup you add, the smoother and creamier the sorbet becomes.
Even small amounts of corn syrup (or other liquid sugars) can add body and creaminess to a sorbet made with sucrose. How much you use, and in what proportion to sucrose, will vary from fruit to fruit, but this lemon sorbet recipe is a good starting point for super-sour citrus.
Oh, and because I know you'll ask: no, honey, agave nectar, and maple syrup aren't good alternatives. For one, they bring strong flavors of their own that may or may not jive with your other ingredients. They're also not very effective; honey has more body than sucrose, but it's so sweet you can't use much of it; maple and agave don't have much body at all.
The article also talks about other ways to soften it (e.g., alcohol), but oddly, only links to a lemon recipe, nothing for watermelon.