I had etter declare an interest first as we are Agents in Ireland for Ravenspring's range of made to measure Dry Suits !
Dry suits are designed to keep you dry ...Not warm !
It is the Mid Later and Base layer that keep you warm and comfortable.
A wickable / breathable mid layer such as a wooly bear or a purpose built two piece mid layer such as we sell will keep you warm. You can also use the layer system of polo necks, sweat shirts etc. Be careful of wearing say a fleece with a high zip as the zip can potentially catch in / compromise the neck seal.
A base layer ( thermal underwear ) if it is wickable / breathable will keep you comfortable and remove moisture from next to the skin. Stops you feeling hot and sweaty in warm weather and cold and clammy in cold weather!
With the exception of windsurf / dinghy sailors, I would strongly reccomend built in boots on your drysuit.
Breathable is a must have if you are regularly out for more than an hour as is an integral fly zip!
When being measured for a dry suit be careful to allow for the mid and base layer underneath!!!