To dive comfortably in colder waters, a drysuit is a must! Most drysuits don't provide much warmth or isolation (especially trilaminate suits), so you'll need undergarments to keep your core temperature up so you can stay down under for longer!
In temperate waters, just an undergarment can be enough, whereas in really cold waters a base layer and undergarment may be the go. Layering up is always a good idea, as it makes your setup more versatile which can be handy for Summer or the shoulder seasons!
