As the weather is too unpredictable for a proper test, I now did a highly unscientific 10 minute test.
Shower + cover = ~tropical rain
I sealed the coat as best I could, and left it in the shower for 10 minutes.
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From what I could find the nylon does repel most of the water.
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There is a slight condensation build up inside, but I would guess this is typical for any air tight fabric, nothing serious. The coat was mostly dry inside. I could find drops close to the openings, but the part where the camera body usually sits was dry.
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It should in other words survive at least 10 minutes of tropical weather
I will try leaving it in the shower for an hour or two, and see if that changes things.