It appears on maps and is barely visible from shore. The maps suggest it's part of the zone. I tried to get there, but didn't find a way to reach it before getting slaughtered. I did get close enough to determine that something weird and interesting is happening there. There is a VERY tall tower on the island, and constant lightning strikes in the area.
You can swim out to the ship that's offshore at Aldcroft, but I found no way to board it.
I do think the slaughterfish are too aggressive. I wasn't even able to circumnavigate Betnikh, or get to the bottom of the peninsula south of Daggerfall, simply by sticking to the shore.
Hircine loves me, this I know,
Your intestines told me so.
Quæ tam fera immanisque natura? - Cicero