You should never operate a boat under the influence of alcohol or other intoxicants. Doing so is inviting disaster and is also illegal in many areas. Alcohol hinders your ability to make good judgment, preventing you from operating the vessel safely. Drinking on a boat can be different from drinking on land as the motion and the vibration of the boat can only increase the level of your impairment. Drinking causes people to lose their balance. This combined with the rocking of the boat can be a deadly combo and increase your chances of falling overboard. Dehydration is also a concern since the sun causes you to sweat, therefore removing water from your body.
You should also operate caution when boarding a boat with unfamiliar operators or passengers. It may not be easy to tell if someone has been drinking or using drugs- experienced users can many times hide their intoxication, however the effects of the alcohol on the body will remain. Always exercise caution when putting your life in someone else's hands.