The HELP and Huddle Positions
H.E.L.P. - Heat Escape Lessening Postures
This position protects the body's three major areas of heat loss (groin, head/neck, and rib cage/armpits). Wearing a PFD allows you to draw your knees to your chest and your arms to your sides.
Huddling with other people in the water lessens the loss of body heat and is good for morale. Also, rescuers can spot a group more easily than individuals.