I had very old hulls that were taking water, with soft spots and all. I couldn't find where exactly they leaked and I decided to change the plugs, without any conviction, just to discard that possibility, and It solved the problem.

Then I replaced the hulls with newer ones (still used). And one of them was taking water again, but now the plugs looked better and were newer. But I checked them anyway, and one of the screws was sort of loose (don't know the transaltion, it's "rodado" in spanish). I took the plug out, and realised that I needed to rotate it and drill another hole for the srew. Put it back, sealed it correctly and, one more time, no water on the hull.