Watermelon is the richest dietary source of citrulline, an amino acid that is metabolized into nitric oxide.

Study: Acute effect of citrulline malate on flow-mediated dilation and serum pharmacodynamics in healthy young males

The study didn't test watermelon, but it showed that when people take citrulline (a chemical found in watermelon), their bodies turn it into substances that help make nitric oxide — which supports the idea that watermelon's citrulline does this too.

Study: Biochemical, physiological, and performance response of a functional watermelon juice enriched in L-citrulline during a half-marathon race

Watermelon juice has a lot of citrulline, and when people drink it, their bodies turn it into another chemical that helps make nitric oxide — which is good for blood flow and recovery. The study saw this happen in runners.

Study: Two weeks of watermelon juice supplementation improves nitric oxide bioavailability but not endurance exercise performance in humans.

Watermelon juice gave people more citrulline, and their bodies turned it into nitric oxide, which is exactly what the claim says happens. It doesn't prove watermelon has more citrulline than every other food, but it shows it definitely works to boost nitric oxide.

Study: Effect of microencapsulated watermelon (Citrullus Lanatus) rind on flow-mediated dilation and tissue oxygen saturation of young adults

This study shows that eating watermelon rind increases a substance called citrulline in the blood, which then helps blood vessels relax — proving citrulline turns into nitric oxide in the body. It doesn't compare watermelon to other foods, so we can't say it has the most citrulline, but it definitely works.