At the ongoing construction of the new Elliott Bay seawall in Seattle, marine mattresses are installed in front of the new face panels to create a shallow water corridor for migrating fish.
Marine mattresses are rock-filled containers generally constructed with UV-stabilized co-polymer geogrid materials. Geogrid panels are laced together to form mattress-shaped baskets that are filled with stones similar to the construction of gabions. They are usually filled off-site and installed by lifting into place.
See video from the Seattle seawall project.