The Giant Deathwatch is only mentioned specifically once in The Incomplete Book of Dragons.
The eye of a Giant Deathwatch is the only mention of this dragon, as seen in The Incomplete Book of Dragons. The eye is blue and the pupil is horizontal, as opposed to vertical, like a cat. No description is given, though it must be pretty big due to 'Giant' being part of its name.
The eye of the Giant Deathwatch appears on a page discussing dragon vision, and is not mentioned again.