Never believe the wattage rating for a head unit. Sure it says that its rated to 200 watts, but in all reality it doesn't put out anywhere near that much. They rate those in a way that the peak output for a split second was able to get to 200 watts, no way its a 200 watt constant output. If you think about the size of a 200 watt amp (50x4), and then think about your headunit, do you really think you can fit the power of that amp, plus all the other headunit items into that space? The answer is no. So, to answer your question yes you need an external amplifier.
Also, just so you know that is telling you that your headunit is a 4 channel headunit and for each channel it can produce up to 50 watts. So even if you hooked up a sub up to it, you can't combine the channels together to power your sub, so the most it would allot is 50 watts max.
If your headunit has preouts (3 sets of RCA cable connections Front, Rear, and Sub) then you want to run that to your amp and then to the subwoofer.
Never go off the ratings posted on a headunit by the manufacturer. They will usually have enough to power to push some lower end midrange speakers, but thats about all they will do.
