I just wanted to post an update. Raw secchi depth data for Chincoteague Bay is published in a report titled "A Water Quality Assessment of the Maryland Coastal Bays including Nitrogen Source Identification using Stable Isotopes." The complete citation is below. I hope this helps other people find what they're looking for.
Jones A, Carruthers T, Pantus F, Thomas J, Saxby T, and Dennison W (2004) A Water Quality Assessment of the Maryland Coastal Bays including Nitrogen Source Identification using Stable Isotopes. University of Maryland Center for Environmental Science (UMCES) Integration and Application Network, Cambridge, Maryland. <http://ian.umces.edu/pdfs/md_coastal_bays_report.pdf>.