Pour salt in a shallow dish for dipping the rims of the glasses. Dip 4 martini glasses in water and then dip the rims in salt. Place in the freezer to frost, about 30 minutes. In a martini shaker shake together the orange-liqueur, Triple Sec, and tequila. Remove glasses from the freezer. Fill the bottom of each glass with the diced watermelon and pour alcohol mixture over the top, dividing evenly among the glasses. Slice lime into wedges and squeeze into glasses. Serve immediately.