Repair broken zippers and split seams. Wear a hood if for no other reason than to seal the neck opening. Heat loss is huge where cold water flows over skin, so a thin full-length suit is warmer than a much thicker shorty. Break the wind. Between dives, a wet exposure suit becomes a swamp cooler.
Many divers actually lose more heat between dives than when in the water.
Wear a parka or a windbreaker, or take off the suit and dry off. Wetsuits are more effective in shallow water. Surface if you shiver. Uncontrollable shivering is the warning sign of hypothermia. This is claimed to relieve nausea.