I shall do some now:
>America
The American navy had reached a nadir following the Civil War. The vast expanse of the American West, and now the American underground, presented a much more attractive opportunity than ill-conceived adventures overseas. The formation of PACT did not help matters, with the Brazilian navy outclassing America's in modernity, size, and funding, and the Mexican navy not far behind. PACT ships now serve as a crutch on surface waters while America continues to "Fuss halfheartedly about with monitors", as historian Lionel Casson would eventually put it. Neither American entrance to Agartha is sea-navigable, further discouraging waterborne expansion below. All in all, it is a miracle that the few American monitors which have been shipped down are present at all. And while they may be nearly twenty years out of date, against Deepfolks and Saurs they hold up well enough. Against waves higher than five feet? Not so much.

>NOTES:
Agartha takes place at the exact turning point OTL where the US started building serious ships again. With Brazil and Mexico (Who did have rather more modern navies at this point) able to plug that hole I don't see an American naval resurgence. Riverboats and monitors are also very cool. Maybe I'll do Germany or UK next. They're both very interesting due to one being more advanced than it should be and the other being the semi-disputed monarch of the seas.