lol, yeti legs, I've got those toooo.
I'm worried about summer coming up myself, I'm incredibly sensitive to the heat so I'm not exactly sure what's gonna happen. Personally though, I have to say I think the thin button-ups are actually better than t-shirts head wise, so long as they're shortsleeved. I wore this really loose, light one last year and it was infinitely more breathable than even the thinnest t-shirts I owned.
I'm scared about swimming, too. Usually we don't do it a lot but we go once or twice and I'm freaking out already that I'm going to be a big hindrance on my friends over it.
Nikolai, what do you wear under the button up? That sounds like a pretty good idea.