https://en.wikipedia.org/wiki/Samosata is the modern city of Samsat https://en.wikipedia.org/wiki/Rhesaina is Ressaina (see http://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:19...) https://en.wikipedia.org/wiki/Zeugma,_Commagene Zeugma is near modern-day Gaziantep
The location of historic Tharrana is apparently not finally known. Harran, Turkey is one possibility mentioned at http://www.euratlas.net/cartogra/peutinger/10_mesopotamia/me....
Since all reference points are in Turkey or Syria, best guess is that the cirular lake with the island is Al Jaboul Lake, 36.024360"N, 37.610087"E. It used to be a tributary to the Euphrates but no source I could find states exactly when that changed.
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.
Edit: seems to be somewhere within the Iraqi marshlands: http://www.indiana.edu/~nelc/events/documents/Jwaideh/jwaide... (back then at least).
I believe it's this.
(It's surrounded by labels for desert, but I can't make out the labels on the island itself.)