The Giant Deathwatch is only mentioned specifically once in The Incomplete Book of Dragons.
Physical Description
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.
Appearances
How to Speak Dragonese
Interestingly, in Book 3, when Camicazi and Hiccup meet for the first time and sword-fight, Camicazi uses as sword technique called the "Deathwatch Parry".
The Incomplete Book of Dragons
The eye of the Giant Deathwatch appears on a page discussing dragon vision, and is not mentioned again.