From examine.com

Quote:To supplement L-citrulline for circulatory health or to alleviate erectile dysfunction, take 1,000 mg of citrulline, three times a day with meals, for a total daily dose of 3,000 mg

(...)

In men with erectile dysfunction as assessed by weak erections (assessed by erectile hardness score[67]) given 1,500mg citrulline daily (two doses of 750mg) for one month was able to benefit half of the 24 patients assessed (as assessed by 'very satisfied' with treatment) while there was merely a 8.3% improvement in placebo.[68]

Citrulline appears to have pro-erectile properties vicariously through arginine. It is likely more potent than arginine at doing this due to having a greater bodily exposure to arginine, but the preliminary evidence right now does not suggest that it is more potent than the reference drug Viagra

In the past I have taken 6gr of L-Citruline as a pre-workout, 3 times a week, but never felt any viagra effect (or any other effect for that matter).
According to the experience above, only 50% of the participants benefited from the supplement and it was assessed after 1 month of daily intake. I'm not sure whether eating a full watermelon once will have any immediate effect or whether you need to eat watermelon everyday. On top of that it does not say if people without erectile dysfunction will greatly benefit from it.

M.