The Collaborative International Dictionary of English v.0.48:

Sea wrack \Sea" wrack`\ (Bot.)
   See Wrack.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Zostera \Zos"te*ra\, n. [NL.] (Bot.)
   A genus of plants of the Naiadaceae, or Pondweed family.
   Zostera marina is commonly known as sea wrack, and
   eelgrass.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sea wrack
    n 1: submerged marine plant with very long narrow leaves found
         in abundance along North Atlantic coasts [syn: eelgrass,
         grass wrack, sea wrack, Zostera marina]
    2: growth of marine vegetation especially of the large forms
       such as rockweeds and kelp [syn: sea wrack, wrack]