You know, those walls they use in subsea cities like New Orleans to hold the ocean back?

If this was paired with a local-sea-level implementation and some way for the game to tell when a body of water becomes disconnected from the World Ocean (pathing system?), a few added structures could create hydrological pumped energy storage, as a high-investment, high-volume alternative to the current molten salt thermal storage. Disconnected bodies of water might even see water levels rise- marginally- when it rains, making dam spillways and/or generator operation a critical part of reservoir maintenance. I wouldn't expect that to provide a significant amount of hydroelectric power on its own, just a technical challenge. Hydrological doors might allow the construction of locks, for ships to access these elevated (or sunken) hydrological regions from the World Map, though that would be a significant technical hurdle.

I would expect water from an overtopping reservoir to flow rapidly downhill, possibly instantly, possibly with erosion simulation, until it finds a low point- or the ocean. I would also expect any void exposed to the ocean to fill instantly, as they currently do, so as to avoid requiring water simulation on top of everything else- simply "here is a pool, how much is in it and what is the water level". This would also tend to make it range from dangerous to terminally stupid to try to "fill in" landlocked water bodies by dumping rocks into them, as it could cause it to overtop and flow into places where you really don't want the water to get.

I'm reasonably confident that implementing a "rain causes mines to fill up" based on this would be a very annoying feature to deal with... but also an excellent difficulty option to increase realism, as real mines have to control the water that gets in- and I understand they deliberately create a low point below the deposit, in order to collect the water for easy pumping. A Mine Pump structure could be the solution to that, with a (slow) wide-area pumping range for limited reservoirs, allowing it to protect excavators as they dig downwards (without requiring the machine to be moved for every tile, the way real-world ones kinda do...)

This idea came from observing that it makes perfect sense for it to be impossible to place retaining walls in the ocean, and noting that there are plenty of walls that are placed directly in the ocean in real life.