The watermelons wont hurt you. They also taste just like normal melons
These are normal whole melons with normal nutritional value? And I can vomit an infinite number of them? For one watermelons are delicious so I could have one on hand at any time. I’d learn to develop my power over time so every melon I vomit would be the most delicious watermelon anyone’s tasted. I’d fake a hydroponic growing operation and start selling them at $30 a pop. Eventually I’d be so good with my power I could vomit a hundred watermelons a minute and with that kind of throughout I could just start using watermelons frivolously. I’d make them into fertilizer, make a plastic replacement from watermelon, use them as landfill to build artificial land in the sea. Watermelon Man would rule the world.
You could hold the world hostage, threatening to cover the world in melons unless they meet you demands. You could train yourself to vomit explosive over fermented water melons that smell bad, like some fierce Birdo. The world could be your subject, constantly in fear of your melons.