Name a common aquatic emergency that requires immediate rescue.

A swimmer actively drowning is a critical situation that requires immediate intervention because it poses an imminent threat to the swimmer's life. Drowning can occur quickly and silently, often without the typical splashing or other signs that people might associate with someone in distress. In this scenario, the victim is unable to keep their head above water, may be in a state of panic, and is unable to call for help. The priority for a lifeguard or rescuer is to respond quickly and efficiently to prevent the loss of consciousness or death.

In contrast, while the other scenarios may warrant attention, they do not carry the same level of urgency. A person with a cramp in shallow water might be struggling but is generally in less immediate danger if they can still keep their head above water. A child splashing while swimming might simply be playing and not in distress. Lastly, a swimmer diving and not resurfacing could indicate concern, but unless it clearly appears that they are in trouble, it might not necessitate immediate rescue. Hence, the situation regarding an actively drowning swimmer is the most urgent and requires a rapid response.