It is a surface area / volume thing. Also, the iWatch has a speaker and microphone, which have to be waterproofed in a different way if they are still to function well.
I've used a few cheap 50M Water Resistant Casio whist swimming without any trouble. I even operated the buttons under water, which the manual said wasn't a good idea.
I think the last non-waterproof watch I had was a Casio Calculator watch - I was nine years old. I then got the Casio Thermometer watch, and I really wanted the Casio IR Remote Control watch - apparently just the thing for upsetting teachers when they showed videos in class.