Open Water Diver is the entry-level scuba certification. PADI and SSI both run it, and millions of people start here. You learn to breathe underwater on purpose, handle the kit without panicking, and dive with a buddy to 18 metres in conditions similar to your training.
No prior diving experience is needed. You do need basic water comfort: a short swim and a float, which your centre checks before you get serious in the water. Theory comes first (often online), then shallow pool or confined sessions where you repeat skills until they feel boring. That is the point. Four open water dives follow, usually over two days, and the certification is yours for life.
PADI and SSI follow the same broad ISO recreational standard, so either card works worldwide. The difference is mostly packaging. PADI eLearning is the familiar route. SSI digital training is flexible, and some centres let you sample an open water dive early. Compare courses on adventuro and pick a centre that suits your diary, holiday plans, or local quarry habit.