As many have said.
Merino wool base layer, I have a variety from different manufacturers.
Then Fourth Element top, Heated vest, and the outer layers are a set of Fourth Element Arctics. I can loose a layer if its too hot.
I use Fourth Element 'hot socks' and then the Arctic Socks on top.
This does allow me to dive year round. Often the heated vest isn't even turned on, I've found it keeps me warm - i think it has a reflective layer.
Dry gloves make a big difference as well. I also have a 'neck warmer', (the type of thing snow boarders wear,) this has made a big difference in the winter. I have silicon wrist and neck seals on the suit, I thing the neck warmer (like 'buff' wear) makes a huge difference, keeping the cold away from the carotid artery.
Most under suits, loose thermal insulation if they are washed too frequently, especially with too much softener. I can wash the base layers as frequently as I like, and they are much cheaper to replace.
Gareth