I'm a six foot man who used to weigh over 330 and I'm now down in the 180s, so I have some perspective on this. I'd like to add a few comments:
1. The plus size ride info at WDWIG is excellent, but pay careful attention to the reader comments if you are particularly large. Most of the attractions are "no problem" in the sense that you won't be asked to get off because the restraints don't fit (an embarassment suffered by many at Islands of Adventure) but lots of them are far from comfortable or easy to get in and out of, especially theater seats. You'll feel much more confident if you know where the potential "minor problems" will be so you can give yourself extra time to get situated.
2. The attractions won't be a serious problem, but heavy people face other problems at WDW, like sore feet and difficulty handling the heat. You need to be prepared for these more than for getting onto the rides. If you don't have a job where you spend a lot of time on your feet, you may not realize just how much your feet will hurt after spending several hours per day standing, for several days in a row. You need to schedule plenty of rest time while in the parks, and plenty of rest time between parks.
3. I'm not about to preach at anyone like some "morally superior" former fat guy, but I would suggest that you use an upcoming trip to WDW as an incentive to get more active and perhaps lose some of the weight. Almost every guidebook suggests starting a walking program several months in advance of the trip -- working up to 5 miles per day -- to prepare your feet for the beating they will take at WDW. That is excellent advice, and paired with a four month commitment to sensible eating, you will be shocked at how the weight comes off. If you do this with WDW as your reward for your hard work and commitment, you won't even realize how great the reward is, because you won't know how much better your feeling than you would have if you had not gotten in shape. I've been both ways, and trust me, the difference is ASTONISHING.
The last time I went the most exciting thing wasn't the ease with which I was able to put on the thrill ride restraints (though that was wonderful). The biggest kick I got was trying to imagine myself walking through the hot parks wearing a heavy coat and carrying my wife on my back the entire time. Thinking how relieved I would feel to put her down and take off the coat made me appreciate how much more fun I was having than I would have been if I was still heavy.
Of course, you'll have the time of your life no matter what, guaranteed! I'm just saying, as someone who's been there, that there is no need to worry about how you'll fit into rides, so you're better off focusing on how you'll feel throughout your trip. Trust me, it's worth the effort!
As for swimming, I figure I am on vacation and I am going to take my kids swimming regardless...I don't let it bother me at all! I love to swim and so do my kids now that they know how! 
