[nkoren]

Good guess!

Here's an alternate proposal: Raqqah. Check it out on the map:

https://goo.gl/maps/ubW2hp1Wh6N2

It obviously isn't in a lake, but it looks like the the tributary coming down from Harran goes both west and east of Raqqah, making it an island of sorts. The shape made by the this "island" is reasonably close to circular. Finally, per the map, it's southeast of Zuegma and south of Harran.

Obviously hypothesising wildly, but this map is just way too much fun.