Even before the MAX grounding only some of the -800 aircraft were certified for the long overwater flights to Hawaii. This article touches on some of the requirements: "Extended-range Twin-engine Operational Performance Standards (ETOPS) certification from the FAA...allows for 'extended operations' over open ocean, something Southwest Airlines has not previously needed." "Flight 6808 was operated on a Boeing 737-800, one of about a half-dozen Southwest aircraft that are ETOPS certified with extra life rafts and additional communication devices." So even if it's in the Southwest long-term plan to fly the MAX to Hawaii it's not something you'll see imminently as it's my understanding Southwest doesn't have any yet that are ETOPS certified.
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