I would say go to Sunset Beach Bar in Maho to watch planes going overhead & onto runway really close
Oh go to Tortuga which is opposite end of Sunset Beach Bar. Prices for food & drink are a bit more reasonable, sometimes less of a crowd. You'll see planes land from either place. Skylinewebcams.com has a listing of cams in SXM of which Tortuga is one right from the outside bar on the beach