if things are too dark you have a few options.
1. Your can slow down the shutter speed. This adjusts how long the sensor is exposed to light. However if youre not using a tripod, or are trying to capture fish it may turn out blurry. A good rule of thumb for shutter speed is if you dont have a tripod, set the shutter speed no slower than the double the sensor size. Example, if your camera has a 35mm sensor set shutter at 1/80s
2. Widen the apeture. This adjusts how much light is hitting your sensor. This is a funtion of the lens, and may be limited depending on your lens. However you may affect the depth of field.
3. Adjust and raise the ISO. This increases the sensitivity of the sensor. However it also cause more noise the higher you go, even with a "noise reduction" feature enabled.
This video is a good brief explanation, as well as the other parts of the series..