How fast would it take for someone with four hundred carpenters and blacksmiths to build a large enough riverrine navy to hold the Trident, assuming they’re using the same methods (assembly line, specialization of labor, standardized parts) as the Braavosi and have access to the raw materials? Would they be able to do it even faster if they could harness hydropower effectively?

Well, the Tullys already have at least some riverrine navy – they send a bunch of ships after Jaime and Brienne.

Honestly, I don’t think it would take that long, especially since the Tullys have a lot of refugee peasants in desperate need of a living. Keep in mind, a riverrine navy has very different needs than an oceanic navy – it’s less about putting together a huge flotilla in one place (since the river isn’t really wide enough for anything like a Blackwater Bay scenario), than having a lot of fast-moving smaller ships that can respond quickly to prevent crossings, patrol potential trouble spots, conduct marine operations, etc. In other words, you’re likely looking at something closer to the longships of the Ironborn than the big 300-man warships of the Royal Navy for the most part, although you’ll probably need some of that to keep the mouth of the Trident/the Bay of Crabs clear.