Hi Lonesailor,
Hobie Nick wrote this a while ago, and I thought he said it perfectly:
http://www.hobiecat.com/community/viewtopic.php?t=3891
When I did mine, I remember thinking "uh oh... is there any foam in here at all???" The drill bit really will just go right through the foam. Like he says - don't push on the drill, just let the drill's weight do all the work.
I think I used a 3/16 on my filler holes (I also found a second filler hole to be useful. Your delam area is probably a slightly irregular shape - just use some common sense when spacing them). The git-rot nozzle fit fairly well in that size. I used the smallest bit I had handy for the breather holes. Probably 1/32 or 1/16?