Both snorkeling and scuba diving are great in Hawaii. Whether you Scuba Honolulu, or venture our of the city a bit, it's really easy to arrange a dive or lessons.
Beginners and advanced divers will enjoy scuba here and the marine life is so beautiful! Beginners can sign up for guided dives that do not require certification. Certified divers can rent any needed equipment, choose a safe spot and off you go! (Tim really likes Shark's Cove in North Shore.)
If you're staying in Waikiki but want to go scuba diving outside the city, most companies will come pick you up for the dive which is very convenient. If you don't have your own equipment, the dive company can provide you everything you need.
Get Certified On Oahu
The Professional Association of Diving Instructors (PADI) is the largest diver training organization excepted everywhere. There are several levels or certification beginning with the most basic that entails three days of training; one day of classroom training and two days of diving. Oahu is a great place to learn to scuba dive and you're sure to see some wonderful marine life during your required dives.