Using a sharp knife, cut off the top and bottom of the watermelon to reveal the red flesh and create flat surfaces on both sides. Stand the watermelon upright, and slice off the remaining peel so only the red core remains.
Cut the watermelon into roughly 1-inch (2-cm) cubes and transfer to a large bowl.
In a small bowl, whisk together the lemon juice, water, honey, salt, and pepper.
While whisking, gradually add the oil in a very slow, steady stream to create an emulsification. Set aside.
Add the spinach, mango, almonds, and lemon vinaigrette to the large bowl with the watermelon.
Toss the salad until well-combined, then divide between serving bowls.