When done carefully, it is perfectly safe. The most frequent problem is someone who is overly anxious to see their penis very large, and goes overboard on the pumping, applying too much pressure to the penis. Applying too much pressure can result in temporary discoloration of the penis, and water blisters. Most symptom usually disappear after a few days. The important thing to remember is to go slow and increase the vacuum gradually over a longer period of time, so that the change in size is not traumatic and does not cause damage to the penis.
Long sessions are better than short high pressure ones.
Warm Water Pumping Never use an electric powered pump or toy anywhere near water. You risk electrocution and may be severely injured or die. Never use water with an electric pump. If you're using a hand pump many pumpers have reported that pumping while in the shower with warm or hot water can be very pleasurable. The warmth may also help the penis to expand.