Rather than do a ludicrously long post (which I almost did), I'll just tell the curious to search (insert search engine of your choice) for "MCO 1500.52B". That is the Marine Corps Order on Combat Water Survival Training. It talks in great detail about what level of qualification is required for different MOS's. It also describes in great detail what is involved with each level of qualification.
I've always thought that making it to CWS-2 was the hardest because you have to do the tows and make the "raft" and pull your buddy around. Once you get 2, moving up to "1" and then on to "Q" is easy. The flight training qualification ain't that easy. But just for basic purposes of being a Marine, if you are comfortable in the water, swim qual is fun. If you are not comfortable and don't think you are a good swimmer, start practicing. If you ARE comfortable and think you are a good swimmer, start practicing swimming with boots on.
Part of the problem with TBS is that you are in the ever popular "Training environment". I made CWS2 in boot camp, and once I got to the fleet, went back for CWSQ. Much easier when there are only 2 people going for Q, rather than an entire company of students. Sounds dumb, but when I made CWSQ I thought it was pretty easy. We'll see what happens in Pensacola, though...