scubadobadoo:Good info here. I'll add, on the topic of personal needs, that I am freezing in my dry suit with heavy undies in 47 degree water. My dry suit helps a bit but certainly doesn't solve the cold problem for me. Now AFTER the dive, I warm up much faster because I am dry! That's the best part! I also don't continue to freeze and thus usually feel like doing two dives instead of one. SO, will a dry suit help? You bet. Will it keep you toasty warm? Probably not. JMHO's. I hate when people say that they felt "toasty." That's just not reality most of the time.
Just to add to your comments, I too am a little chilly by the end of a dive but it is possible to be "toasty". I'm not but I have been. It wasn't worth the trade off to me. You can wear a heavier and heavier undergarment until you are toasty the whole dive but the increase in weight required to do this is not worth it and therefore a compromise is usually settled upon.