I find it easier to hold my breath while swimming. I can easily swim a lenght of our public swimming pool underwater without air (I haven't timed it, but it's a large pool, I'm guessing it's more than two minutes) but I simply cannot stay still and hold my breath, I think it's like what Frasier described in
Perspectives on Christmas, the physical action distracts you
Like I say, I can swim without comming up from air, but I tried just ducking underwater and I didn't make 20 seconds!
Mind you, I have mild asthma, so maybe that's it, although it still doesn't explain why I can do it longer while swimming.
On a slightly different note (blinking), I did find this one trick, it was really scary, a few years back, I found if I sat in a certain position, and leaned my chin on my hand, and I put my fingers in a certain point in my mouth, I suddenly didn't need to blink! I timed myself and went 10 minutes without blinking! I tried it and I outstared my mum in a staring contest by more than twice her time! However, when I went to sleep that night I seemed to forget the position, and despite trying several time, I haven't been able to do it