Captain Webb is an agent with British Naval Intelligence.
This is in itself interesting because Webb is a captain in the British Army, not the Navy. Specifically he is a member of the Royal Engineers having been a civil defense engineer prior to signing up.
The time in which his recorded adventures take place is 1941, two years into World War II though a few months before America would join the conflicts. Webb would come to the attention of the Navy when he took it upon himself to assist in a matter that used not only his engineering expertise but also his personal hobby of underwater diving.
It was after a German bombing attack on London in which an unexploded bomb became lodged underwater at the base of a vital rail bridge. Webb had heard a radio report of the problem and had raced to the area to see if he could be of assistance. Using the diving equipment he had created himself, he had located the exact position of the explosive and soon figured out how to disarm it.
[It is important to note here that historically this would be taking place one year before Jacques Cousteau's breather apparatus would be unveiled and more than a decade before the more modern concept of SCUBA gear would come to be. Of course, this adventure was penned in 1966.]
Two weeks would pass before a meeting of the Deputy Chiefs of the various armed forces would take place to discuss a problem removing a German aqueduct. In that meeting it was felt that a Naval Intelligence operative was needed to go into enemy lands and see to the destruction personally since bombing runs had so far failed. Since that person would need to have a good understanding of how to optimally set the explosives, it was the navy Admiral who would recall reading of Webb's achievement and suggest him. Suddenly the civil engineer turned Army captain was onboard a submarine heading to Germany.
We learn after a time that one of Webb's areas of engineering expertise was in underwater structures which explains his knowledge of working in that environment and his self-designed and self-made equipment. Webb, described as a "tall, broad-shoulder man with keen features", enjoys his time beneath the surface, though likely not as much when having to deal with enemy frogmen or submarines.