What is the name of your state (only U.S. law)? IL I thought up the idea of a watermelon dispenser when I was 16 years old and years later I saw on YouTube that Vat19 YouTube Channel stole my idea for a watermelon dispenser and was selling it, also Bed Bath and Beyond stole my idea for a watermelon dispenser and was selling it too, so do I have a legitimate argument to sue both Vat19 YouTube Channel and Bed Bath & Beyond for infringing on my watermelon dispenser idea?
There are an assortment of watermelon "dispenser" patents. Obtaining a patent can be the best way to prevent others from profiting off any original (novel, non-obvious) product idea you may come up with.
For information on patent protection and the patent application process, you can visit the US Patent and Trademark Office website at http://www.uspto.com.
You can also potentially protect an idea by establishing yourself as the originator of it. You can do this by disclosing and discussing the idea with a company/manufacturer after securing signatures on a non-disclosure or confidentiality agreement and then working to develop the idea. Having documentation showing dates and content of discussions can be important should someone decide to take your idea as their own.
That said , you cannot claim rights to an idea if you have not made any effort to protect and develop it. From what you have said so far, it appears you have no legal action to pursue against Bed, Bath and Beyond, or Vat19, or any other entity that is selling the watermelon dispenser product envisioned by you when you were 16.