Scoop or cut the watermelon from the rind and puree the flesh in a food processor or blender.
Strain puree into a large bowl to eliminate seeds and pulp.
In a small saucepan, heat 1/2 cup of watermelon juice with the sugar and salt, stirring until sugar is dissolved.
Remove from heat, let cool a bit, and then combine with the remaining watermelon juice, lime juice and tequila.
Cover the mixture and refrigerate until chilled.
Pour mixture into an ice cream maker a freeze according to manufacturer's instructions.
Once frozen, transfer into a freezer-safe container a freeze until firm.