Seagrass: 

The Seagrass Ecosystems Research Laboratory of the Florida International University.  Primary emphasis is on seagrass studies and ecology of Florida Bay Ecosystem.
http://www.fiu.edu/~seagrass/

Western Australian Seagrass Web Site.
http://wwwscience.murdoch.edu.au/centres/others/seagrass/

Seawood:

Algae Base

http://www.algaebase.org

International Phycological Society
http://www.intphycsoc.org/default.lasso

Journal of Phycology
http://www.psaalgae.org/res/jphyc.shtm

The Phycological Society of America.
http://www.psaalgae.org/

Provasoli-Guillard National Center for  Culture of Marine Phytoplankton at the Bigelow Marine Laboratory.
http://ccmp.bigelow.org/