While that's totally correct about SJU not being international, I remember that when WN first started flying in there there were reports of atrocious load factors into and out of SJU. SJU has improved a bit since then.

Flight loads can be searched here: http://www.transtats.bts.gov/DL_Sele...Air%20Carriers

The most recent data is from April so it's hard to draw conclusions (I don't believe that there is any Southwest branded international flying).

A few snapshots (April LFs, all on FL branded flying as there is no data yet on WN branded flying):

MCO-AUA: 73.1%, AUA-MCO: 92.6%

AUS-CUN: 75.4%, CUN-AUS: 82.9%

ATL-NAS: 62.4%, NAS-ATL: 69.1% (I believe WN has already dropped this route)

SAT-MEX: 75.5%, MEX-SAT: 74.1%

SNA-MEX: 78.8%, MEX-SNA: 83.7%