When wearing a dry suit you will need an undersuit of some description to provide your body with insulation from the water. Depending on whether your dry suit is neoprene, crushed foam or membrane determines what level of undersuit you will need. Thicker neoprene dry suits require less insulation under them as the neoprene itself provides protection from the cold, whereas a trilaminate or membrane suit will require more as the suit itself has less insulating properties. Ultimately however all humans are unique in their ability to fight the cold and you need to try a few variations to get a level of comfort that suits you.
There are 2 main types of undersuit. There is the one piece undersuit and a two piece suit. It really is down to personal preference, but if you get a decent version of either you will be nice and warm.
I dive membrane dry suits and have various undersuits which i vary depending where i am diving. I feel the cold. I'm a skinny guy and need to be protected from the cold waters i dive in. I hate being cold in the water. Given the choice i will dive dry whenever possible; from the cooler waters of Ireland to the much warmer waters of Egypt.
WHITES THERMAL FUSION
FOURTH ELEMENT XEROTHERM
FOURTH ELMENT ARCTIC
I use these in moderate temperatures around 16C.
MERINO WOOLEN TOP
Ok, a quick explanation of what merino wool is. It comes from a particular breed of sheep and has certain properties which lend itself wonderfully to diving.
It regulates the body temperature when worn against the skin, it has anti bacterial properties, provides warmth without causing overheating and has "wicking properties." Wicking refers to the materials ability to draw moisture away from the skin, i.e. sweat. This in turn keep the wearer from feeling wetness against the skin.
I have a merino wool top which i wear every dive. It stops moisture build up on the other layers i have on and reduces the smell that all undersuits seem to have after a few dives! Don't worry, all divers stink!
I use unlined dry gloves, which, if used on their own my hands would get cold quickly. So i have to wear a glove liner underneath. I have 2 pairs of gloves that i wear depending on the water temperature.
EDZ Thermal Fleece Gloves: These are a motorbike glove liner but i find they work just as well for scuba. They are made from 165g microfleece and are very light and very warm. If the water is especially chilly i wear these over the top of my merino or silk gloves. Dexterity is reduced but your hands will always work better warm and bulky than cold. They also dry very quickly and are easily washed.