Why do you feel tired after swimming?

Answer: Whole body works harder against water resistance

Whole body works harder against water resistance ✓ — Correct! Swimming works almost every major muscle group at once, arms, shoulders, back, core and legs, and every stroke pushes against water, which is far denser than air, so each movement costs more effort than the same motion on land. Keeping your body horizontal and breathing in rhythm adds to the load. The exact energy you burn depends on stroke, pace and your own size, but a steady swim is a demanding whole-body workout, which is why you feel it afterwards.

Cold water drains energy to keep you warm — Cool water does make your body spend energy on staying warm, and that adds to the tiredness in a cold lake or sea. But people feel worn out after a session in a warm pool too, so temperature is a contributor, not the main cause.

Holding your breath uses up energy — Good technique means breathing in a steady rhythm, not holding your breath, and controlled breathing is only a small part of the effort. The fatigue comes from moving your whole body through water, stroke after stroke.