You do not need to take the filter apart.
Any space in the fuel tank above the fuel level will be air. This will contain moisture, as does the air around us. When the temperature is cold, the moisture condenses on the sides of the tank, and drains down into the fuel. The fuel rises above the water, so some water is drawn forward to the filter, which is designed to separate it out. When the water volume gets to a certain level, the light comes on to tell you that you should have drained it more frequently. There will be a tap on the bottom of the filter to drain the water out. Anything drained out should be disposed of properly. Not down a drain, or onto the compost heap, etc, but taken to a garage to add to their waste oil is the best idea. (How many people do that?) Your handbook will advise on location of drain, and how.
Petrol vehicles also get condensation, but the water gets pushed through with the fuel and just reduces the power a little. Less of an issue. The diesel filter has to separate it as the injectors and high pressure pump rely on the fuel as a lubricant.
Ideally, all diesel vehicles should have their tank filled to the brim before every cold night. Bus companies are good at this, as they have the facilities and routines. Many large hauliers can do so too, but a pain to individual vehicle owners.
This problem also occurs in the storage tanks at garages, although less so as they are underground tanks and their temperature is more consistent. Good filling stations will regularly dip the tank with some water identifying paste on the bottom of the dipstick and water is pumped out. The fuel delivered to your car at garages is taken from the tank a little above the bottom, so you can never get the last of it out via the pumps.