Use a sharp knife to carefully cut the ripe watermelon into uniform 1-inch cubes on a clean cutting board. Arrange the cubes on a serving plate or place in a mixing bowl.
Cut the avocado in half lengthwise, remove the pit, and peel the flesh. Slice the avocado into thin, even chunks or scoop with a spoon for a rustic texture. Add the avocado pieces to the watermelon on the plate or bowl.
Drizzle the lime juice evenly over the watermelon and avocado; this enhances flavor and prevents browning of the avocado. Sprinkle a pinch of sea salt over the mixture.
Gently toss the watermelon and avocado with a spoon or fork to combine, ensuring the lime juice and salt are evenly distributed. Be careful not to mash the avocado; aim for gentle mixing.
Transfer the mixture onto a clean serving plate or keep in the bowl ready to serve. The final dish should showcase juicy watermelon chunks with smooth avocado accents, visually vibrant and appealing.