IMHO...Since you are also looking for nightlife and are alone, I wouldn't stay at the north shore. I'd stay in town (Waikiki). Then if you have transportation you can drive up north to surf. If you won't have a car, then this is also your best bet for surfing. Can't say much about where to surf since I go where the waves aren't.
But as for scuba diving, most shops do hotel pick ups in Waikiki. If they aren't diving the south shore, then they will take you either to east side or more than likely the west side where diving conditions are more predictable.
As for dive shops, there are many good ones. As an instructor who taught on Oahu for 7 years, make sure you ask questions. This will make sure you're doing the type of diving that you want, and are comfortable with.
1 - Do they own the boat or are they going to buy a seat for you? (Many shops all book on the same few boats so you wind up with 3 or 4 different groups all on the same boat)
2 - Do they know for sure (conditions permitting) where they will be diving on the days you want to go out?
3 - Since you're not from Hawaii are they guided dives?
4 - Will you be buddied up with someone with about the same amount of experience as you?
5 - What ammenities (creature comforts) are there on the boat? For example, if they advertise that the boat only holds six divers, there may not be a marine head (toilet), camera table, easy access to water, food etc.
I would suggest looking at
Ocean Concepts. They do a great job with visiting divers. But definitely ask questions about all the shops before you book a trip. Most will also offer multi-day discounted rates if you are going to be diving 3 or more days.
Have a great time, I loved my time on Oahu. The nightlife in Waikiki sometimes doesn't get going until 11:00 at night so you can even get in a
night dive before hitting the clubs.
Aloha, Cheers and Safe Diving,
R