If you're in the US, there is lots of WQ data at the NERRS stations. Perhaps one of these coastal areas will have relevant data. http://cdmo.baruch.sc.edu/
In the Long Island Sound area (if that's where you're interested) there's the Long Island Sound Study, the CT Dept of Energy and Environmental Protection, and the Interstate Environmental Commission. They've been monitoring Long Island Sound for temperature, salinity, and dissolved oxygen for years. CT DEEP may also have data on phytoplankton abundance.
These articles may be useful " BureaucracyEcological Indicators, Volume 11, Issue 1, January 2011, Pages 103-114Paul Bierman, Megan Lewis, Bertram Ostendorf, Jason Tanner" and " Water Science and Technology, Volume 40, Issue 2, 1999, Pages 151-157A.U. Mahajan, C.V. Chalapatirao, S.K. Gadkari" and " Estuarine, Coastal and Shelf Science, Volume 76, Issue 2, 20 January 2008, Pages 418-430Fedro U. Tapia González, Jorge A. Herrera-Silveira, Maria L. Aguirre-Macedo"
National agencies should have this information and it should be free. For you NOAA will defiantly have this data alongside that the EPA will have some coastal data.
See the below link it may be useful to you, click the any blinking location first, now location map comes; later you click the view data. Then you should get physicochemical data.
For the Indian River Lagoon, FL, Harbor Branch Oceanographic Institution is working on a real time monitoring system that is publicly available. http://fau-hboi.loboviz.com/