When the carbon dioxide is very low, you will not feel the need to breathe before the oxygen is too low, and you will get unconscious. No need to tell you that this situation is extremely dangerous. When you go up, the partial pressure of oxygen will drop, and this again causes unconsciousness. Here again, we don't understand why people do it. Pushing your limits of apnoe diving causes extreme physical discomfort.
Alcohol reacts differently under pressure.
This is the bottom of the list. Some marine creatures know how to defend themselves with venomous spines, stinging cells or teeth. None of them will swim towards you in order to attack you. As long as you don't touch them, they won't harm you. Some fish, like big triggerfish, are territorial.