How to make sorbet from fruit like watermelon?
Some suggestions in the event that the questions doesn't get closed as a duplicate...firstly, get a copy of McGee's book, as suggested in the first possible duplicate.  It is an excellent resource with too much information to reproduce here. Referring to McGee, you will want to decide what kind of texture and what level of sweetness you desire. The proportion of sugar in the mix and how fast it is frozen impacts the final consistency. It's not really about pulp content. Again, this is all outlined in McGee's excellent book. What McGee calls "moderately sweet ice" (20% sugar) becomes rock hard when frozen, and needs to be used before that state...or allowed to temper after.  Sweet ices contain about 35% sugar and need little, if any thawing.  Creating the desired texture is a matter of balancing acid and sweetness given a specific ingredient.  Watermelon and lemon will behave very differently, and McGee has a table that helps to see why.  The ratio of sugar to acid in lemon is 0.4.  For Watermelon it is 45. For example, he suggests the following for a sweet lemon ice: 1 C juice, 14 TBS sugar, 1 cup water.