There are many good clinics in SD where you can go in case of emergency: Cedimat, Abreu, Dominico-Cubano, Corazones Unidos, etc. You can find a full list on any local medical insurance company's website. Since there is no ambulance service, just take a taxi to go to the nearest good private clinic.
Cedimat has the best equipment maybe, but that doesn't mean that you get the best help there in case of emergency, that depends on what happened to you, a heart attack or a broken leg.
Travel (for the first 90 days) insurance is OK in any private hospital, BUT this is something when they try to make bills as high as possible and do some unnecessary procedures for that, so keep in mind that you have to ask about every procedure, do you really need it or no. For example, my friend from Europe was on vacation in Cabarete and he had appendicitis symptoms. The surgery was done at some hospital in Puerto Plata and the bill to his Czech insurance company was 12.000 dollars. And he was like a hostage in the hospital until they paid it.