On Roatan you'd probably be good in West Bay. Nicest beach on Roatan although the cruise excursions know it also. Some nicer restaurants and some beachfront places also.
I don't have a specific resort recommendation but some that get mentioned frequently are:
Infinity Bay
Los Rocas
Henry Morgan
Bananarama
Mayan Princess
All have on-site or adjacent dive operators. No shore diving there though - for that you need one of the AI dive resorts - most have little or no beach.
Some others that just might work across the island - all of these are more isolated to some extent:
Barefoot Cay
Media Luna
Parrot Tree Plantation
Turquoise Bay
Two miles from West Bay in the West End is where most of what passes for nightlife on Roatan is. And there's a water taxi between them.
I don't know northern Belize at all. Some friends stay in Placencia in Southern Belize where there are some excellent beaches. Downside there is the reef in most cases is an hour boat ride out so that limits the amount of dives that can be done daily as well as the time used up during the day to do them. It's really popular in late spring/early summer when the Whale Sharks feed off Gladden Spit - the only place in this hemisphere where diving is allowed.
Here's one location maybe not on your radar - Curacao. It's second only to Bonaire in shore dive sites. Almost all of them are also nice sandy beaches with dive operators and some sort of food option on site as well. Beach operators charge a small fee but that always includes a chair and often an umbrella or thatched palapa when the g/f tires of the sun. At many you should be able to find dive buddies also - there's also several guided options including the Dive Bus or Bas Harts Diving.
My go to place there is the Marriott Emerald Beach/Casino resort. Nice beach, nice divided pool complex/bar, Caribbean Sea Sports on-site for both boat and shore dives. They lead a guided night dive once per week also. It's really as nice as it looks in the pictures. It's two miles north of downtown which is just far enough to be quiet yet provide easy access. The cruise port is a little south of there also but they go south away from the resort to downtown's famous shops etc. The diving is really easy, the visibility is often exceptional and it's hard to get lost since the reef parallels the shore line.
There are other options on Curacao also with all the amenities. Lions Dive Resort has everything and is a few minutes south of town. Not exactly the quietest part of town with a beach club and the Sea Aquarium adjacent. Ocean Encounters main shop 20yds. away is their house operator.
Curacao, a unique Caribbean island paradise. Official Curacao site
Grand Cayman would work also except it's pricier. There's about 2 dozen dive operators and most of them pick-up twice/day at any of the Seven Mile Beach properties which is a nice beach but popular in certain areas. The Westin or the Marriott are both nice but in a more crowded area of the beach. If you don't mind a condo, some of the complexes on the beach on the north side are quieter - and the beach is often wider. Several of the marinas where the dive boats are kept are just across the street so it's pretty convenient. There's also more reasonable off-water properties like Sunshine or Comfort Suites. Both have beach access via a short walk.
There's also about 6-8 shore dives on the west side of the island and guides are available at most of them. At Sunset House, the Dive resort south of town, finding a buddy to shore dive there wouldn't be a problem - most people there are divers. No beach though so not the best place for you both to stay. That area and the area north of Seven Mile Beach are mostly all ironshore down to the waterline and beyond.
Cayman Islands Official Tourism Website | Welcome to the Cayman Islands
Hope this gives you some ideas.
