COOL DOWN
Cool down [also known as warm down] think of this as helping you recover ready for the next training session.
The objective is gradually slow things down. This achieves improved removal of waste products that can cause soreness.
Reduces the likelihood of blood pooling especially to the working muscles. Provides better supply and for longer of blood rich in nutrients including oxygen and the removal of carbon dioxide. Lack of proper warm down can result in short tight muscles.
A cool down should be 5-20 minutes below 65% of maximum heart rate [HR] for that particular sport [max HR can be higher by 30 bpm compared to swimming]

Cool down static stretching is a good time as muscles are warm, being wary at first as muscles may have tightened after the workout.
Aim to hold the static stretch for 15-30 seconds and repeat twice, see if you can stretch a little more each time without any pain. If short of time complete at least one for each muscle used or aim to stretch once for 30 seconds rather than twice for 10 with 10 seconds break.
You may feel that the cool down takes up too much time, try and see how much they help with better recovery and improved muscle length.