Prepare the watermelon by cutting it into manageable chunks on a cutting board using a sharp knife; remove seeds if desired.
Add the watermelon chunks into a blender, then pour in the cold water.
Secure the lid on the blender and blend on high speed until the mixture is completely smooth, about 30 seconds to 1 minute. You should see a uniform, vibrant pink liquid with no large chunks remaining.
Place a fine mesh strainer or cheesecloth over a large bowl and pour the blended mixture through it, using a spatula or spoon to press the liquid through and separate the pulp from the juice.
Once strained, transfer the clear, pink juice to serving glasses. Optionally, refrigerate for a few minutes to chill further before serving cold.
Serve the watermelon juice immediately, observing its transparent appearance, crisp aroma, and refreshing taste.