Not sure what's going on, but this code:

from siphon.catalog import TDSCatalog
thredds_url_hind = "https://tds.hycom.org/thredds/catalogs/GLBy0.08/expt_93.0.xml"
cat_hind = TDSCatalog(thredds_url_hind)
ds_hind = cat_hind.datasets[0].remote_access()

fails due to no OPeNDAP (or any other service) url being generated in access_urls. Manually clicking the link on the HTML catalog page gives a set of access URLs that seem to work fine.

Taken from this Stack Overflow question.