No, you do not need to know how to swim for scuba-diving. But you need to use a lifejacket at all-times. And try to fin around a swimming pool with the lifejacket, fins, mask and snorkel for at least an hour before jumping straight into the ocean. Because no matter how confident you are, if you have not been inside water, you will PANIC.
Case 2: Scuba Diving for a one time experience:
You need not know how to swim for this, either. The instructor/DM will give you half a day pool/confined water sessions before taking you to the ocean. And they will hold you the entire time during the dive.
Taking Scuba Diving As A Hobby
Theoretically, you don’t need to know swimming to be able to dive. But, there are several considerations, which makes it necessary to be able to swim before diving.
1) All of the organizations (PADI, SSI, etc.) require you to pass a mandatory swim test (not something physically demanding) during your Open Waters(which is the start of your lifelong scuba diving adventure). It is to make sure thatyou are comfortable in the water.
2) Many dives require you to swim away from the beach/jetty before you reach a reef or a wreck. You will miss those dive sites.
3) Very often, it happens that the sea conditions are rough, and the currents very strong (sharks always prefer to stay in regions having strong currents). For these dives, you need to be a good swimmer (to be confident about the pitch, apart from the experience to be considered as well).
4) Overall, you need to know how to swim, for your safety as well as everyone around you. Plus, you would be comfortable with the dive and enjoy it more.
On a side note, you might find people arguing (a lot less though) suggesting that diving is an equipment-intensive sport, and all you will ever do is ‘fin’ around, but you would be very wise to ignore them.
Is Swimming And Scuba-Diving Same?
Swimming and Scuba Diving may seem similar, but they are different in ideologies. While swimming, you use both your hands and legs to move while breathing fast or holding your breath. But when it comes to Scuba Diving, you should only move your legs and breath at a very moderate pace (short inhalation and long exhalation through your mouth, and never hold your breath). You are expected to save as much energy as you can (never overexert yourself underwater). That is a lot of difference right there.
Since you’ll be wearing something called the BCD (Buoyancy Control Device) Jacket, and wearing one of those, it is impossible to drown. With the BCD on, it is you who decides whether to stay afloat or sink (by controlling the amount of air that you fill into the jacket), unlike swimming, where you depend on your body and skills to stay afloat or sink. You’ll learn about all these techniques, jargon, and acronyms in detail when you join a course.