Who is William McCullum and the Sinhalese Divers from Hornblower and the Atropos and what is their importance?

McCullum is a salvage expert. He is in charge of the underwater demolitions, in which the Sinhalese divers recover large amount of gold and silver from a sunken ship in Atropos. McCullum is described as a large, muscular man, who is gruff and difficult to work with.