Yep, definitely DEH-koo. I always said DEE-koo as a kid, but I was corrected several years ago by a fellow fan, heh. It's hard not to slip up and say it wrong out of habit.
I always pronounced it "Deh-Koo" but recently I've been watching a lot of Zelda lets plays and seem to have picked up a habit of pronouncing it "Day-koo". It seems to differ depending on where you live and your accent.