"What have I done to deserve this?" My dive buddy howled at the universe. Neither of us is particularly superstitious, but we were beginning to wonder what higher power had decided to keep us from diving. Already, we had hiked out into the jungle, braved a ridiculously thick fog of mosquitos, and splashed into suspiciously warm, brown, and pungent water. We were floating at the mouth of a cenote, in what was basically a mosquito factory, when we found bubbles coming from all three of his regulator first stages and one of his pressure gauges. To make matters worse, his light wouldn't turn on, and his drysuit was slowly filling with the impressively smelly water. This was Not Our Day.
Occasionally, the wisest decision a diver can make is not to dive. When multiple pieces of gear are failing and the odds seem stacked against you, they are. My dive buddies and I follow the Rule of Three - when three things go wrong, we cancel the dive. Yes, we might be able to fix everything and go on the dive, but after three problems, we are usually so frustrated that our heads are not in the game anymore. Instead of entering the water stressed, rushed, or irritated, we prefer to take a break and regroup. It's better to get out of the water (or off the boat), fix the problems, and return for a fresh start at a later time, or even on another day.
Equipment failures and logistical problems cause delays, and create time pressure. Last minute quick-fixes to gear do not always inspire confidence, especially when they involve copious quantities of duct tape and zip ties. I have witnessed plenty of divers make shoddy or incomplete repairs to dive equipment because they did not want to cancel a dive. Many last-minute repairs are made in the field with out proper tools or repair parts.
Dive gear is not comfortable out of the water,and many times equipment failures are discovered once a diver has donned his gear. Even failures that occur before a diver is suited up usually require a diver to deal with the problem in a less than perfect environment (such as a rocky boat deck). Environmental factors, such as heat, cold, sun, wind, sea-sickness, and in our case massive swarms of abnormally large and blood thirsty mosquitos, raise a diver's stress level and may cause him to rush through equipment repairs. Once a diver feels stressed, his mental control and focus is gone. When a diver becomes unfocused and stressed, he is unlikely to enjoy his dive and more likely to make mistakes. This is not a good state in which to start a dive.
More advice for safer diving:
Many divers use the Rule of Three. Having a specific point at which problems cause you to cancel the dive makes sense because it takes pressure off of the diver with the problems. He doesn't have to admit that he is stressed; he simply invokes the rule of three. This rule also helps divers to avoid hyper-focusing on the goal of diving beyond what is safe or even rational (such as duct-taping your fins back together, zip-tying your mask on, jumping in the water despite the fact that your BCD won't hold air). In our case, the decision not to dive was difficult - we had invested a great deal of time and effort to arrive at the dive site - but it was the right one. You can always dive another day.