Hello Johann Claassen
Well, possible ancestors of your Grandfather certainly enjoyed a popular name, all over the UK, and on the Isle of Man!
If you do a free search on findmypast (UK) you will find many males with the surname 'Waterson' and first names similar to Henry John, or William Henry J etc etc.
On IGI, there are a couple of centuries of Henry Watersons. Of course variants in spelling of the surname of Waterson, also is prevalent.
The Henry Waterson on the Isle of Man, who may possibly be your Grandfather's ancestors, hail from different villages and towns there, such as;
Henry Waterson, b. 1824, Peel, Isle of Man
Henry Waterson, b. 1840, St Anne, Isle of Man
Henry Waterson, b. 1845, Patrick, Isle of Man
Henry Waterson, b. 1866, Conchan, Isle of Man
Henry Waterson, b. 1876, Douglas, Isle of Man
Henry Waterson, b. 1879, Arbory, Isle of Man
There are many more going back a couple of centuries.
Any of which could be your Grandfather's relatives?
It may be worth contacting the Births, Deaths, Marriages for Isle of Man and seeing what help you can receive in a search for your Grandfather's, parents' possible marriage, based maybe on guessing his father is born about 23 years, before your own grandfather's birth, in 1900? And, giving your Great Grandfather's names as the same as your grandfather's?
Maybe your Great Grandfather is THE Henry Waterson, b. 1876, Douglas, IOM?
Good luck!
Danchaslyn