Personally, the reason I go to SXM over Hawaii or west coast Mexico is because of the beaches, swimming pools are there if you don't have a good beach.
Cupecoy area beach comes and goes but it is a great location that allows easy access to the rest of the island and has Dany's right on the beach as well as several restaurants and bars in the area. Mullet not a long walk and it is the most active beach on the Dutch side.
French side has some places that are lively but not a big evening bar scene, more of a dining scene.
My three suggestions other than staying in Cupecoy which is what I would do.
A place that has a mix might be The Morgan, good pool area and close to a lot of the action. No beach right there but not far away.
Sonesta - not my cup of tea but it does seem to have a lively pool area and beach on site as well as a good beach a short walk away at Mullet.
Atrium - Traffic can be an issue in that area around 3 to 6pm and the pool is weak, beach not the best but you have a lot in the evening with in walking distance including my favorite evening bar, Huppel the Pub.
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