Flylink® 2 Million Pixels CMOS 5m / 16ft Waterproof USB Endoscope Borescope HD 6 LED Tube Pipe Inspection Camera (7 M) (Electronics)
Easy to use, you can even connect a smartphone (be careful, because of the power consumption that may not work ...) and then use an app with "USB camera". As stated in the comments the cable is not very rigid, but anyway, push the camera through