Have you ever been knocked over by a wave? Imagine spending much of your life being smashed by them. Waves only affect the surface. Deeper in the sea the waves have no effect on the lives of living plants and animals.
Where the sea meets the land it becomes shallow. It causes waves to rise and when the wave reaches the beach, rocks or a cliff it smashes into it. That means any seaweeds or animals will also be crunched.
Most seaweed and many animals survive these waves by glueing themselves down or holding on tight to rocks. Their fronds or bodies must be very strong or well protected.
Glueing can’t be done in shifting sand and holding on tight in sand won’t work if the waves are strong.