Salt water and ocean currents
Waves breaking on the shore - seawater contains far more salt than most habitats on land.
Sea water contains about 35g of salt in every litre of water. This would be a problem for land-living organisms, but not for sea creatures.
Because levels of salt and other chemicals are much more consistent than in fresh water and on dry land, the environment is very stable for life.
Currents in the water are more difficult to deal with. There are some deep ocean currents that circulate water around the globe, but most movement occurs near the surface, as waves and tides form. Shallow-living organisms have to be able to cope with these unstable conditions.