To answer the question if you should wear a swim cap, let's first think about why you are swimming and to what purpose would the cap be to you. So why do you swim?
If you are out there stroking from one side of the pool to the other for fun or a wonderful exercise and you have short hair, buying a swimming cap could be an overkill, therefore not necessarily something you'd need or enjoy. The only thing you need is a swimsuit and some decent goggles and you are set.
On the other hand, if you have long hair or are thinking of becoming a competitive swimmer, a swimrunner or a triathlete, it could be beneficial for you to wear a swimming cap once in a while and spend some time researching the types of caps that are out there.
1) Do you have long hair? If so, I'd suggest you do wear a swim cap as it is very hard to swim with the right technique, with the correct head position while trying to lift your head to keep the hair out of your face.
Also, some folks say that using a swimming cap does prevent your hair from chlorine damage, but I wouldn't think too much about this. Chances are, you don't spend 40hrs a week in the pool for your hair to get damaged anyway, so you are safe there.
2) For the group of you that will be swimming in competitions and you don't suffer from long hair metal mania :), I'd suggest you DO NOT wear a swim cap in your regular swim practices on most occasions.
The reason for this is that you want your time spend in the swimming pool practicing to be harder and with more drag than while you are racing. And most importantly, you negate some of the feeling of water flow over your head if you wear a swim cap all the time.
Your head without a swimming cap has the hair and ears flopping around, so the drag is much higher. If you work harder in practice, just by having more drag, your racing will feel much faster. So, to keep your drag larger in swim practice, stay away from swim caps.
Not to be confused with minimizing resistance while swimming.
3) There are times in swim practices, however, that you should swim at competitions speed and practice certain things you will do in your swim meet, triathlon or a swimrun competition.
For example, practicing relay starts, relay exchanges, starts, turns or plain all-out sprints. During these activities, you might want to consider wearing a swimming cap as it gets you used to the feeling which you will have while competing.
When you swim with a swimming cap, your body and head feel a bit differently than without a swimming cap.
Because you have less drag with a swimming cap, you will glide further in streamline positions and could go much deeper into your starts etc.
So, practicing these with a swimming cap once in a while, before you go to your main swim competition is a good idea.
4) If you are an open water swimmer or do swim often in very cold water, it is a very good idea to wear a swim cap as it helps with retaining your body heat.
Most of your body heat usually escapes through your head, so by placing a swim cap on it will keep you warmer for longer.
On the other hand, you can also easily become overheated in a warmer pool, so be careful.
Swimming out there in the open water could be dangerous, especially if there are boats and other water sport athletes around your area. To decrease the chance of a collision with someone who might not see you, brightly colored swim caps should always be worn.
5) If you think swim caps are uncool and you are out there to make a fashion statement by not wearing a swim cap, then think again as swimming caps come in a huge variety of colors, designs, materials, you can even design your own swimming cap, so don't worry about the way you look and get the cap that serves your purpose.
As mentioned above, there are swim caps made from many different materials. They range from lycra to latex and to silicon.
If I have to give you just one piece of advice about swimming caps, DO NOT buy Lycra caps. Pardon my french, but they are just dumb.
First, they add more drag, they will not stay on your head and they will definitely not help you reduce any drag.
For everyday swim practices, it is better to invest in latex or silicone caps. Latex caps are cheaper, but they do not last very long so it could be better to invest in silicone type cap.
However, be careful that the cap is not very tight on your head as your ears will start hurting after a few minutes in the pool.
From my personal experience, I'd suggest Latex caps for practice swims or a lower quality silicone caps which are less tight.
Nothing sucks more than ripping a new swim cap on your first day because your fingernails were too sharp :).
The below video can help you with the proper technique on how to put your swim cap on.
Finally, if you are after a minimal drag in the water, the newish no-crease seamless swim caps, which are very smooth, might be the way to go for you.