Seaweed, or sea grass
Archimandrite Nikifor’s Biblical Encyclopedia
(Jonah 2:6) — sea, water grass, as it is read in the Russian translation in the citation given. Seaweed is mentioned in the prayer of the prophet Jonah to God from the belly of the whale: “the weeds were wrapped about my head.” There are very many kinds of it; some are of microscopic size, while others reach gigantic dimensions, up to 200 sazhens in length. The green color of our standing waters depends on the enormous multitude of small, green, freshwater algae, which sometimes cover the surface of the water like a thick felt.