Halve the lemons lengthwise and slice to about 1/4-inch thin, so you have thin half-moons of lemon slices.
Mash together the lemon slices, sugar, and salt until the sugar dissolves and the juice thickens.
Strain as much liquid from the lemon slice mixture as you can (use a spoon to help separate the juice from the solids in the strainer).
Add the watermelon juice and 2 cups of the water to the lemon juice mixture in pitcher. Taste and add more water as necessary until you are pleased with your lemonade base.
Add the habanero pepper. Refrigerate overnight. Remove the habanero when the lemonade reaches your desired heat level (I'm wary of overpowering the lemonade flavor, so I take it out after about 24 hours). Add chunks of fresh cut watermelon and sliced lemons to the pitcher before serving.
This is a forgiving recipe. If you want to intensify the heat, leave the habanero in the lemonade longer. If you prefer a more subtle heat, remove it sooner or just use half of the pepper. If you accidentally leave it in too long, dilute with more lemonade. To up the sweetness, add more watermelon or sugar. To make a boozy version, add vodka or tequila. If you can't find watermelon juice, you can instead replace the juice with water and add 1/4 of a watermelon, cubed, to infuse into the lemonade.