I ice dive and I'm quite happy in a membrane drysuit, Viking to be specific. I don't have to wear a huge heavy undergarment. The temps really aren't that cold to be honest. I think
36 degrees might have been the coldest I've seen. It's obviously possible to get colder if you stay up top next to the ice, but once under it's pretty tame and you don't need anything heavier than what you would ...