The main resort in the South used by the UK TOs appears to be Bayahibe.
The 2 main hotels there were the Iberostar Hacienda Dominicus (IHD) and the Sunscape Casa Del Mar (CDM). Both have long threads in the forum.
The CDM reportedly changed hands late last year but there are a few forum members due to take holidays there shortly so no doubt they will report back.
When we went on a day trip to Santo Domingo we stopped at a few hotels further along the coast so there are other alternative but I am not sure what their beaches are like.
All the trips to Saona Island leave from Bayahibe itself so we decided to base ourselves there and chose the IHD. They did have a "steak house" but I was not too impressed with that meal.

Juan Dolio and Boca Chica are also in the south.

The former was badly hit by a hurricane some years ago sustaining a lot of damage to the hotels and beaches, I don't think its ever really recovered but there are some acceptable hotels there at a reasonable price, don't expect luxury though. The hotels run along the beach with a service road to the front and private dwellings in between, there are some small shops, restaurants and bars scattered about too. The good points are that this makes it easier to leave the hotel and have a wander about, you can also get a bus up on the main highway, something you can't as well in some of the more isolated hotels in the DR. I would advise that you research hotels here well, some leave a lot to be desired.

The beach at Boca Chica is awesome to look at but it is a public beach and so gets very busy at weekends with locals and can be a problem for the ladies, the younger and prettier the worse it is, but no female is immune, even those with a husband in tow. There are also not many hotels actually on the beach, only one as far as I can remember. We stayed at the Dominican Bay hotel some years ago on a cheap last minute deal, although not beach side, it does have a walkway of bridges that lead you down to the beach and their own club house there. Then it was basic and a little tired but excellent value for what we paid. The downside is that not many British TO's sell packages to Boca Chica, the nearest airport is Las Americas about 8km away, the capital, Santo Domingo is about 30km making it possible to do a DIY holiday if you're brave enough, again, being a town its possible to explore locally and not be confined to the hotel.