Neoprene underwear (innersystems), neoprene tops and shorts provide extra protection from the cold when worn under a wetsuit.
Neopren underwear (innersystems) are not only useful for winter to provide even better cold insulation, but are also a useful addition during the rest of the year. For example, you can sometimes use a thinner wetsuit on cooler days, if the winter neoprene has already been winterized or you can simply wear the underwear alone on hot summer days. The neoprene undersuits are usually between 0.5 and 3mm thick. There are different variants such as long sleeve, short sleeve, with hood or without hood.
Neoprene tops and shorts can also be worn under a wetsuit or of course alone in the summer as UV and cold protection.