With the exception of Enteromorpha, the high seas traveller, and floating Enteromorpha, which occurs on quieter waters, practically all larger algae are anchored to the shore or shallow bottoms. The oceanographer would probably call them "land weeds" instead of "sea weeds." Nevertheless, they are bathed with sea water most or all of the time and so they partake of the ocean, even though the zone may be only a narrow one. While their total productivity is small compared to that of the entire ocean, yet, area for area, it can be very high.
Blinks, L. R.. 1955. "Photosynthesis and productivity of littoral marine algae." Journal of Marine Research 14, (4). https://elischolar.library.yale.edu/journal_of_marine_research/849