John McCready is a diving expert.
We learn a lot about him in this short passage given us early in our introduction to the character: "In his early forties, fifteen of which had been in saturation diving, McCready lived for the job. It had seen a marriage come and go, and time since then had been in total dedication to the work. It was, after all, not a profession you could take lightly."
As for description, around the same time we are given a pretty good description of the man with: "He was just under six foot tall, with broad shoulders and a rugged, weather-beaten face that had been around the world and back again. He had piercing blue eyes and a hint of stubble that was occasionally allowed to flourish into something greater, but at the moment was tamed to a mere shadow across his face. Behind the subtly handsome features, lay a quiet yet steely determination that was rarely prone to anger, but that right now could well be put to the test."
From the description of his profession, it is obvious that McCready is not and would not want to be an agent. He had found his life's work and was happily doing when life steps in the way and pushes him into that kind of work though only because he is so good at being, like I said, a diving expert.
With so much of this planet comprised of oceans and the need to transit those waters so important for trade and security, it is not surprising that many a times comes when vessels do not make it to their intended destination and instead sink far beneath the surface. With those oceans being so deep and working in them so treacherous, when items vital for national security disappear below the waves, the need of the Intelligence communities shift from an operative to a diving expert, like McCready. Whether he wants it or not.