Hello, i have found a way around the error.

I went into my Local files again and deleted certain ones. After deleting the entire second expansion folder and having the launcher re-download that particular one, the Underwater section is now accessible without problems or the error. The Red sea area and the MSQ are available to me again.
The exact path i took was to go into steam, open the Local files, select the "game" folder and then "Sq pack" the folder then displays 5 sub folders. In my case i deleted the "ex2" folder for the stormblood expansion.

I may be theorizing here but if similar issues appear in other expansions, subsequent or before Stormblood, a similar approach could be taken should Errors like this Happen.
Thank you for your support