What do you need to swim?

Depends on where, when, how long, and how deep yopu want to go. A short while, in fairly warm water - just hold your breath and go under. Swimming goggles or a diving mask will let you see better.

A longer stay just below the surface, use a mask and a snorkel. If water gets cold, use a wetsuit, or a dry suit if it's really bad.

If you want to go deeper, use Scuba gear.