An alternative Earth in which Queen Victoria still reigns after 160 years and Magna Britannia rules the waves. This is the setting for the Pax Britannia series, of which this is one of the most recent. I haven't read any of the other stories in the series, but that didn't spoil my enjoyment of this one. The hero, dandy and British agent Ulysses Quicksilver, accompanied by his trusty and resourceful valet Nimrod, refers to various previous and picaresque-sounding adventures but none have any bearing on the story of Leviathan Rising.
The plot concerns the maiden voyage of a huge and luxurious new submersible cruise-liner, the Neptune, able to visit exotic underground cities as well as more conventional resorts. An assortment of VIP guests, including Quicksilver, has been invited along, but their pleasure is soon spoiled as one of them is murdered. Not long afterwards, the Neptune sinks out of control to the sea bed, and Quicksilver has to use all of his resourcefulness and courage to save the steadily dwindling band of survivors from various fantastical perils of the deep while solving the mystery of who is guilty of murder and sabotage.
A novella, Vanishing Point, is included in the same volume (from Abbadon Books). This features the same hero, this time involved in a country house séance mystery which turns out to involve international espionage.
I gather that this sub-genre is known nowadays as "steampunk", although the term which coined itself in my mind when I read it was "retrofantasy". If you enjoy Jules Verne, H G Wells or the adventure thrillers of Arthur Conan Doyle, you'll love these tales. The first retrofantasy I can recall reading was Harry Harrison's 1972 novel A Transatlantic Tunnel, Hurrah! written very much in the style of the period, as the title suggests. Another comparator from the 1960s and 1970s (also set in an alternate Victorian-style but contemporary world) would be the Lord Darcy tales by Randall Garrett, which differed in including magic. More recently, I can recall reading the 1990 novel The Difference Engine by Gibson and Sterling, about an earlier development of computing in an alternate Victorian England.
I'm not a particular fan of this sub-genre, which seems to have an appeal (presumably greater for British than for other audiences) for a time of confidence and certainty, when technology marched relentlessly forward, nothing seemed impossible, and (of course) Britannia ruled the waves. However, I enjoyed Green's stories; good, old-fashioned entertainment with a nostalgic air.