Mangroves and seagrasses typically do not grow along the beach primarily because 2) they cannot take root in the shifting sands.
Beaches are often characterized by loose, shifting substrates, which make it difficult for these plants to establish stable root systems. Mangroves, in particular, prefer more stable sediment and typically thrive in intertidal zones where the sediment is more stable and can support their root structures. Seagrasses, while found in shallow waters, typically grow in more stable substrates and not directly on sandy beaches.
The other options, such as water depth, salinity, and temperature, could influence their growth but are not the main limiting factors for these specific habitats.