50% of the heat lost on a dive is from your head. One of the best ways to increase your comfort and warmth on a dive is to add a hoodie.
The alternative is to add extra warmth to the core by adding vests and or hoodies.
Most vests are a 3mm neoprene and can go both under a wetsuit or on top of the wetsuit adding an additional layer of warmth for the diver over the core.

Don’t forget that if you add neoprene to your diving configuration, you will need to add some extra weights.