Many of the West Coast - Hawaii flights (to all islands) are and have been operated by 737s for decades. Has nothing to do with Southwest, as UA was operating 737s long before Southwest started service to the islands. Surely, you won't get a 737 on what's considered the long-haul Hawaii flights (DEN and all points east), but yes, they're mixed in with some widebody service on SFO/LAX flights, and on other O/D points again like SNA-HNL.

CALMSP 's advice is accurate...Try to switch to another flight with widebody service--if you were on a widebody before, that's a good enough reason to call in and move to a 'better' aircraft.