Add the watermelon and cucumber chunks to a blender and blend until thoroughly pureed. Pour into the sieve and allow the juice to slowly drip into the bowl without forcing any of the pulp through. Discard solids. This should yield about 12 ounces of juice.
In a pitcher, combine the watermelon and cucumber juice with the lime juice and tequila. Fill 4 highball glasses with ice and evenly divide juice among glasses. Top off each glass with seltzer water. Serve the glasses topped with a couple of cucumber and lime slices.