This is the in-situ decay of rocks/regolith exposed to the atmosphere. There are three branches of weathering: mechanical/physical, chemical and biological.
These largely depend on the type of climate at the coast.
Freeze-thaw: This tends to occur in climates where the temperature fluctuates between above and below 0 degrees Celsius. Water enters cracks in rock, and when temperatures go below 0, it freezes. When water freezes, it expands by 9%, so occupies more space and exerts more pressure on the rock. When temperatures rise, it melts, allowing more space to fill again. The process occurs many times before eventually the pressure is so much, and the crack has expanded so much that pieces start falling off.
Temperature change weathering: This happens in places where radiation cooling can take place (very warm then sudden cooling). During the day, heat makes rock minerals expand. Sudden cooling at night then causes them to contract. The repeated expansion and contraction causes stress along mineral boundaries. Eventually, layers start to peel off.
Wetting and drying: This happens in climates with both wet and dry periods, at quite regular intervals, and often, the process works most effectively on clay. When water is added to the rock, it expands. As it dries out, it contracts. Constant expansion and contraction creates cracks in the rock, making it more susceptible to other weathering, like freeze-thaw and salt crystallisation.
Salt crystallisation: This tends to happen more in warmer climates because more evaporation will be able to occur. Sea water enters cracks in rocks, and then evaporates in warm conditions. As it evaporates, it leaves behind salt crystals. As salt accumulates over time, it increases the pressure on the rock, leading to it cracking and breaking off.
This is common on coasts because rocks are very exposed to air and moisture.
Hydrolysis: This will normally occur in a climate with rain. Water can become slightly acidic, so when it comes into contact with rock, it combines with minerals in rock to create clays and dissolvable salts. This degrades the rock directly, as well as meaning that the products are generally weaker than the parent rock, meaning it is more susceptible to further weathering/erosion.
Hydration: This occurs in damp/wet climates. It is the physical addition of water to minerals in a rock. It makes the rock expand, leading to stress which can then cause the rock to disintegrate. It also weakens the rock in general, making it more prone to further weathering, chemical processes in particular.
Carbonation: This can happen in most climates but tends to be most effective in cooler temperatures (water can dissolve more carbon dioxide). Carbon dioxide dissolves in rainwater in the sky, producing a weak carbonic acid (H2CO3). This acid reacts with calcium carbonate (CaCO3) in rocks such as limestone and chalk, producing calcium bicarbonate (Ca(HCO3)2). This dissolves easily in water.
Oxidation: Water can have oxygen dissolved in it. The oxygen reacts with certain minerals in rock, creating oxides and hydroxides.
This includes processes which lead to breakdown of rocks by the action of vegetation and coastal organisms. Certain marine organisms, like the piddock (a type of shellfish) have adapted shells which enable them to drill into solid rock. Seaweed can attach itself to rocks, and sea action causes it to sway, which can be enough to loosen parts of the rock, so it comes away. Organisms like algae release chemicals which cause/promote solution. Animals may burrow into the ground which can contribute to damage and weakening of rocks. Roots growing down into soil for nutrients and water can go through rock joints, widening them so causing a build up of pressure.