Winair also has an interline agreement with JetBlue.
Note that US airlines often require that both the US airline and Winair flights be booked on the same reservation. That’s best done by booking the entire reservation through the US airline website/app, and not one reservation for the US airline and a different one for Winair. I no longer live in a JetBlue city, and I cant remember the last time that I flew United, so I haven’t looked to see if that applies here. (If you do have different reservations for the US airlines then you can try asking the checking agent if they can help you, which sometimes works. No guarantees though.)
Tradewind has interline agreements with American, JetBlue, and United. That may not have much value on the way home, as you have to recheck your bags after clearing Customs in SJU, but it could be helpful on your way to SBH.