海草(うみぐさ)refers to ”seagrasses”, flowering plants that grow in shallow waters, such as Zostera spp, not the marine algae indicated by the word "seaweed".
May I recommend that when you wish to refer to seaweed you use the characters 海藻 (かいそう)?
This is particularly relevant to Okinawa where dugongs graze on seagrass, NOT seaweed.
More information is available here:
http://en.wikipedia.org/wiki/Seagrass
日本語の説明はこちらに豊富にあります:
http://ja.wikipedia.org/wiki/海草
Thank you.
よろしくお願いします。
サイモン