Keep in mind wearing a drysuit can cause you to tire easier and quicker than normal. You are more restricted with your movements because of the bulk, therefore, it takes extra effort to maneuver.
To help you determine if you should wear a wetsuit or drysuit, use this Wetsuit - Drysuit Temperature Chart.
A good rule of thumb to follow for safe waterskiing is the "100 degree rule." This means the air temperature plus the water temperature should be greater than or equal to 100 degrees F to be skied comfortably. You can certainly ski in temperatures lower than the combined 100, however, most people feel that it becomes uncomfortable at that point.
One of the biggest things you have to watch out for in cold water skiing is hypothermia. Hypothermia is a general cooling of the entire body. The inner core of the body is chilled so the body cannot generate heat to stay warm. In cold conditions, your body will concentrate keeping your torso warm and put less emphasis on getting blood to your extremities.
Water conducts heat away from our bodies 25 times faster than air. Also, the winter months often bring windier conditions than normal, creating a wind chill factor. And to add to this wind chill, you also have the speed of the boat creating stronger winds. The windier it is, the faster heat is conducted away from your body. This can cause the body temperature to drop quickly. This wind chill table gives a graphic representation of the relationship between ambient air temperature and wind velocity.
Several things can lead to hypothermia. Of course, the number one thing we are talking about here is cold temperatures. Amazing enough, a temperature of just below the normal body temperature of 98.6 degrees F can cause hypothermia. Others things that can increase your chances of getting hypothermia are insufficient clothes, dehydration, exhaustion, recent consumption of alcohol, and lack of food nutrition.
Things to be aware of when acknowledging a person is suffering from hypothermia are impaired motor skills, pale skin, drowsiness, confusion, slurred speech and uncontrollable shivering. If these symptoms are evident take measures to reduce heat loss. If an extra change of dry clothes is available have the person change into them immediately. It's always smart to pack an extra set in cold skiing conditions. If no change is available, cover the person completely with other means of layers like towels, sweatshirts, or hats. A dry life vest will also do. Have the person move around to increase blood flow to warm the body. Position the person in the boat as to where they are shielded from wind.
Give the person non-caffeinated, non-alcoholic, hot liquids, sweets, carbohydrates, and proteins and fats. Avoid nicotine. Put the person in front of a heat source. If none is available, with the engine off, open the engine cover to absorb heat generated. Share the body heat of other people on board by grouping together under towels.
The second biggest thing to watch out for in cold water skiing is frostbite. This is the freezing of some part of your body. It is distinguishable by the hard, pale, and cold quality of the skin that has been exposed to the cold over an extended period of time. The area is likely to be numb, although there is likely a sharp, aching pain. As the area thaws, the flesh becomes red and painful. Any part of the body may be subject to frostbite, but the hands, feet, nose and ears are most vulnerable.
The first sign of frostbite is a prickling sensation followed by numbness and hardening of the skin. Once this occurs, shelter the victim from the cold and wind and remove any wet clothing. Unless you are to a place where you can keep the frostbitten area warm and thawed, do not rub the affected area. Refreezing can cause further tissue damage. Huddle up with other people on the boat to share body heat until you reach shore.
Another tip - bring a big cooler full of hot water. It is great for soaking gloves and headbands before a run and for a quick dunk of hands and feet both before and after hitting the water. Lots of towels to keep dry help as well (tip submitted by Ken Cram / John Hirsch).
For further study try the book Hypothermia, Frostbite, and Other Cold Injuries by Wilkerson. This compact, comprehensive book covers hypothermia causes and effects, and tells how to prevent, recognize and treat it.
Bottom line, don't be dumb and ski numb!