Why is my camera taking really dark pictures?

Hi Utuado

I dont't know if this will help, but here is somethings that could contribute to the problem you are having.

The light sensor should be faulty or dirty. But a dirty light sensor should give you lighter pictures as the camera should compensate.

The settings could be changes so you have a higher Exposure value (EV) resulting in darker photoes

There could be dirt or sediment on the camera sensor, blocking the light from being detected. You could follow the guide for repairing the autofokus, Repairing Nikon D3100 Autofocus

as you also here has to clean the sensor.

I hope some of these issues could be the cause of our problem :)

If the camera had indeed gone into lake/river water, the first thing I would do is to take it to a technician for a thorough cleaning. Without a doubt. Get it cleaned and tested.

this sort of error message is consistent with the autofocus motor fault, the autofocus motor system, youd need to remove the bottom panel of the camera and clean the autofocus motor cogs and apply a little grease to lubricate them. that would be the fix had the camera not went for a casual swim in the lake. id recommend taking it to a camera specialist for a once over, this i'm afraid may cost more than the camera is worth to fix, there may likely be more than one fault here. good luck.

I'd guess that the aperture lever in the lens has been either broken off, or knocked out of alignment. This would mean that if the lens is a G-type lens, it will always be at minimum aperture and the lack of aperture lever will mean it won't be opened up when it's mounted on the camera. For more hacks check Nikon D3100 Manual.