DL is flying direct on some days of the week, and connecting via Seaborne through SJU on others. Immigration and Customs in SJU is a dream compared to MIA. United does something similar from EWR.

There are really only two airlines who are fully committed to SXM with daily flights - JetBlue from JFK and American from Miami. Hate AA as much as you want for changing flights to a connection through the hated MIA, but at least they're operating 7 days a week.