Amarynth,
Really interesting situation, thanks for sharing. I don't have any specific advice, but have the following thoughts. Remember that swales, raised beds, etc are just techniques, they are not principles nor stand-bys, they are only useful in appropriate context (climates/landforms/etc). So go back to the principles and gather baseline data.
Swales have many functions, one of which is to slow overland sheet flows of water, which causes erosion. Are there obvious erosion examples? If so, swales might still be useful.
Raised beds help plants grow in the tropics when the soil is water logged. Does the soil become water logged in the rainy season? If so, they might still be appropriate.
Maybe we can think of those underground rivers as your aquifer or water table. Water is still being held in the soil, plants are still drawing water from the soil, and I doubt they are taking it directly from the underground rivers. Although that is a good question that I don't have an
answer to. So, manage water to minimize erosion and manage soil to increase its water holding capacity.
As for the plunge pool, my gut tells me that would be harder, but again I don't know the context. Maybe you'll have to go above ground with ferrocement.
Hope that helps at all. Please continue to share!
Saludos
Scott