There are so many ways you could explain the swimsuit.
1. He's a robot and sees the swimsuit as a way of reasoning his ability to breath underwater.
2. It's some sort of advanced swimsuit and reacts as soon as it detects water on the skin, covering him up.
3. The swimsuit has little glowing bits on it, that makes it look like the structure gel covered machines. Maybe the structure gel grows and adapts to protect Simon/Robot Simon from any changes in environment. That was what the stuff was made for.
Also, do you ever see simon swim? I know you see him outside in the water, but does he swim or walk/drag up ledges? If he was a machine like the others, I can't really see him doing that. They're not exactly advanced enough looking for terrain and ocean adventures. They look like chunky industrial enginerring machines. The machines that we've seen moving in the water could be designed purely for that purpose. He's either a human in a swimsuit or some specially built kind of machine. Like others have said, an android.
As for seeing his hands, being out of breath and hearing his heart beat; we've all seen from the trailers and other snippets, that the machines who think they're human maybe hear/see those things as well.
Truth be told, we could speculate until the cows come home on what exactly Simon is.
We won't know until the 22nd September