I usually eat a normal (for me) breakfast, but I've never been able to hit the water any sooner than several hours after that. It's not comfortable to dive on a very full stomach, but it's also distracting and tiring if you're too hungry. I try to split the difference.
I have puked through the reg, and I was pleasantly surprised at how uneventful it was. Thanks for the tip about not ascending while puking.
I have been seasick before, but not terribly, and never while diving. I've heard that it goes away when you get in the water, but most people are reluctant to begin a dive when everything's not right. No surprise there.