The Black Sea is huge, even sonar sleds have fairly narrow widths, so it would take a ton of work to find potential wrecks, and you need to visit all of them to

The Black Sea is huge, even sonar sleds have fairly narrow widths, so it would take a ton of work to find potential wrecks, and you need to visit all of them to determine which ones are modern and which are older. And that's a very long and costly process since it's well below diving level and very dark. You have to send down an ROV which can basically see only a little in front of itself and in a narrow field of view.