If you are talking about pre-emergence as in before the watermelon seeds come up but a total kill on all vegetation then round-up (Glyphosate). It kills everything. It does not hurt any vegetation planted after kill since it is only absorbed through the leaves. When it is on the soil it will not be taken up into the plant. Though you cannot say you are organic if you use it.
Roundup is not a pre-emergent herbicide since it needs the plants you spray to be fully mature and growing. Don't mow before you apply roundup because it relies on leaf surface to take the herbicide to the roots for a total kill.