Adding area places with categories "Sea" and "Island" seems to be the fastest solution so far, but it's not the ideal one. The ideal solution is to report to your Global Champ or Coordinator who will escalate to the Staff Community Manager.

If the cost appears to be drowned with water on Live Map use an "Island" area place to cover the cost, or use "Sea" if it's the other way around. You can be as detailed as you want but keep in mind that the app will automatically reduce the geometry points. Only Live Map will show the detailed geometry.

This is what I've learned from most communities that have this problem and I started using it in Saudi Arabia. I advise to discuss this with your local community to agree on a unified practice.

Thanks for your answer. I learned that Waze is not handling reports of wrong coastlines promptly. Requests can take long time to be handled. I need to hear from concerned staff at Waze about a practical way to handle such issue in an acceptable manner.