What is the term for coastal defence walls that extend into the sea?

The term for coastal defence walls that extend into the sea is breakwaters. Breakwaters are structures designed to protect the shore from the impact of waves by creating a sheltered area for vessels and reducing coastal erosion. These walls typically extend into the sea and help to calm the water near the shoreline, providing safe anchorage for ships and reducing the energy of incoming waves, which helps to protect the coast.

Groynes, on the other hand, are barriers built out from the coast into the water, primarily used to interrupt the flow of sediment along the beach but are not primarily defensive structures themselves. Sea walls are vertical or sloping structures built directly along the coastline to absorb and reflect wave energy but do not extend into the water like breakwaters. Jetties are structures extending into a river or sea to influence the current or tide and are mainly used for navigation purposes rather than for direct coastal protection.

Thus, breakwaters are correctly identified as the coastal defence walls that extend into the sea.