Diving Suits help protect the diver.The sea/ocean has tremendous vectors and unaccustomed environment and objects.The diver ,and specially the delicate human skin is liable to injuries if not properly sheilded by specialised garments and protectors!
Heres where the diving suit gets preeminence.
Even nanotechnology has come to the being and even rescue.Nano bodysuits are the future!.Now the body suits employed offer adequate protection and security to the divers.

The different types of suits -
Dry suits
Semi-dry
Wetsuits
Dive skins!
hot water suits
Now most of these suits are made up of Neoprene and heavy duty material coated with rubber and PVC!
TO increase the buoyancy or floating ability ,which in turn is dependent on the volume of the suit,and also weight.Newton!! Density/mass/volume!!

Sometimes a Weightbelt is worn to counteract this buyoncy.SOme drysuits have added mechanical regulators that allow the user to regulate the buoyancy by allowing the amount of trapped air to be reduced (usually during ascent.