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I think it is pretty clear what Marlee is looking for. A good time. A party break from school and home, some cool people to hang with and the ability to be 22 without being hassled or overtly targeted. When she says she wants to come and live, she is really saying she wants to come and experience living here for a few months then go home. I call that an extended vacation. "Culture" to Marlee probably equates to the practical day to day realities of staying here on the local economy without the support structure of a resort and tour guides.
The reality as I see it, is that there are not too many places in the DR where a foreign young person can slip in and spend a couple of months and find everything they are looking for and expecting. Not a lot of young people choose to do this here. Young-ish persons into wind sports do choose Cabarete for the wind and the waves. If encountering people my own age is an important consideration, that's where I would focus my attention. Cabarete is smaller than Sosua and after a trip through the meat dept. in Janet's Super Mercado, I would not choose to live there. Sosua has two grocery stores to choose from. As for the prostitution, it shouldn't be much of an issue for a female traveler.
I would think Sosua has more available in the way of short term rental accommodations but that is only my opinion. I am sure great places are available in either town. English is important to Marlee. Even with lessons, her Spanish won't be good enough to live outside an area where tourist English is available.
I recently dissuaded someone I know from coming to Sosua for a month. I know this person well, and bluntly told them they will be bored after 5 or six days. There is nothing to do here in Sosua except drink, eat, sit on a beach and repeat. Those of us who live here rely heavily on the daily grind of living to fill the available daylight hours. I cut the grass, buy groceries, weed the garden, exterminate the insect pests, wash my moto, rake the leaves, clean the pool fastidiously, wash the dogs until all their hair falls off and shop online. By the time I have done all this at a slow tropical paradise pace, it's dinner time and I can usually find something to watch on TV until bed time.
If the majority of these activities were not necessary, I'd go out of my mind with boredom. Even with them, I do get a little bored with the routine on occasion. There is nothing to do in Sosua to make a month fly by like the week or two on a resort tends to. One (even young people) can only party so much, eat so much and sit in the sun so much.
As for true culture, and to keep herself sane and to avoid the potential for long term liver damage, maybe Marlee would consider volunteering with an organisation that works with local children, or animals or some other charity a few days a week. Marlee would get to help and maybe make a difference while here and with few other choices, this is a good way to purposely tick off some hours and to soak up the reality (culture) of the DR.
I am sure Marlee could handle the 15 minute guagua ride between Sosua and Cabarete or vice versa frequently. It wouldn't really matter which town she ended up staying in. I'm sure the trip into Puerto Plata would also be no problem when a change of scenery becomes necessary.
I don't know what constitutes "True Dominican Culture". Sitting on the side of the road watching the world pass by? Playing Dominoes sitting on the patio of a convenience store sipping cold ones and arguing about everything? The two big cities in this country offer lots of diversity of both people and activities but for the most part don't satisfy the requirements of an extended vacation premise.
Marlee will find it difficult to bump into North American or European 20 somethings who will be here for as long as she plans to stay. She will meet people, they will leave and she'll have to start over again with the next bunch that arrives.
Is her plan do-able? Sure it is.
Is it safe to do: Yeah, but like everywhere, if you put yourself into dodgy situations sooner or later you'll encounter grief.
Will she have difficulty meeting people: Not a chance. At 22 she'll be the focus of interest/attention for every Dominican male and expat alike.
For a month or two sure come on down. Make sure someone always knows where you are, where you are going and when you expect to be back to your temporary home again. Plan on spending some time alone and lots of time meeting and dealing with new faces. Be smart and be very weary of Dominicans, especially the men, who develop an unhealthy interest much too quickly. Be aware of the pervy gringos too.
Good luck Marlee. I hope you choose to come. Just be prepared to leave when you've had enough.