1. kelp: a giant plant sometimes 60 metres long with no roots, it is anchored by hold-fasts or tendrils up to 10 metres long, that cling to rock. Gas filled bubbles on fronds act as floats keeping the kelp just below the surface. (Earth Sciences References, Mary McNeil)
  2. sea weed: general name for marine plants of the algae class which grow in long narrow ribbons. Also called sea grass. (International Maritime Dictionary, 2nd Edition)
  3. sea grass: any grasslike marine alga. Eelgrass is one of the best known seagrasses. (IHO Dictionary, S-32, 5th Edition, 4565)
  4. sargasso: a certain type of sea weed, or more generally, a large floating mass of this sea weed. (IHO Dictionary, S-32, 5th Edition, 4501)