I however believe this to be a living creature. Could this be a Black Boa like the one sighted last year in Iceland? My estimations from looking at the proportions of the boat and the creatures head is that its head is at least 2 metres wide, which would mean the body would be at least 170ft long. A true sea monster indeed!
Where is this creature now? Has it gone to aestivate? We know it prefers river water to salt so could it now be in the River Foyle?
Its very likely there is another of these creatures in the area as they travel in pairs.