a sea race like this sounds more fitting for antiquity to me: imagine if the fate of rome and carthage hinged on who built the biggest and most powerful ships faster
That actually happened IOTL
But the OP was talking about building bases and whatnot under the sea.
pre 1900 POD
The Deep Sea Race happens between Spain and England:
Everything starts with Blasco de Garay who gets more money from the Spanish government for his inventions, which (well, at least not a lot) didn't happen in OTL
Blasco de Garay (1500–1552) was a Spanish navy captain and inventor.
De Garay was a captain in the Spanish navy in the reign of the Holy Roman Emperor, Charles V.
He made several important inventions, including diving apparatus, and introduced the paddle wheel as a substitute for oars.
Garay himself sent the emperor a document setting out eight inventions which included:
- A way to recover vessels underwater, even if they were submerged a hundred fathoms deep, with only the aid of two men.
- An apparatus by which anyone could be submerged under water indefinitely
- Another device to detect objects on the seabed with the naked eye.
- A way to keep a light burning underwater.
- A way to sweeten brackish water.
Spain develops a tradition of underwater exploration for prestige and the search for lost gold.
England the Spains rival doesn't like to be behind so they seize their opportunity in form of Cornelis Jacobszoon Drebbel
He builts the first navigable submarine in 1620 (like OTL) while working for the English Royal Navy. Using William Bourne's design from 1578, he manufactures a steerable submarine with a leather-covered wooden frame. Between 1620 and 1624 Drebbel successfully builts and tests two more submarines, each one bigger than the last. The final (third) model had 6 oars and could carry 16 passengers. This model is demonstrated to King James I in person and several thousand Londoners. The submarine stays submerged for three hours and travels from Westminster to Greenwich and back, cruising at a depth of from 12 to 15 feet (4 to 5 metres). Drebbel even takes James in this submarine on a test dive beneath the Thames, making James I the first monarch to travel underwater. This submarine is tested many times in the Thames and starts a contest between Spain and England about who can build the most advanced under sea technology.
One of his most important discovery is a way to re-oxygenate the air inside one or more of these submarines (still OTL). He generates oxygen by heating nitre (potassium nitrate or sodium nitrate) in a metal pan to make it emit oxygen. That turns the nitrate into sodium or potassium oxide or hydroxide, which tends to absorb carbon dioxide from the air around. That explains how Drebbel's men were not affected by carbon dioxide build-up as much as would be expected. He accidentally makes a crude rebreather nearly three centuries before Fluess and Davis did in OTL. Drebbel had been taught by the alchemist Michael Sendivogius (1566–1636) (perhaps when both were at the court of Rudolf II) that warming nitre produced oxygen.
The race inspires literature genre of Deep Sea Tales. They mostly resolve around fighting giant seamonster, newly rich seaweed plantations owner, lost cities, and under water fortresses. One of the more famous example is the book
Bioimpulsae, a thinly veiled satire about republican ideas, starring an expy of Niccolò Machiavelli.