There are two ways to jump into a cold pool of water.
The first is to walk into it slowly - toes first, then feet, then ankle, etc. This allows each of your body parts to slowly get used to the temperature, all while experiencing what you were most afraid of in the first place for a longer period of time.
Or you could just jump in. This allows you to experience the water quicker and with less effort. It also makes for a much more exciting and memorable experience.
Don't think about it and just jump in next time.