In responsding to your question on classifying in COFOG sea walls/defenses to protect against sea erosion, we have sought advice from various experts and their views differed significantly.
In your incoming query you stated that the sea walls were not similar to breakwaters and thus not related to water transport. Further you indicated that the purpose was not land improvement under environment protection.
As a result of the consultation process, we arrived at the following conclusion:
In COFOG expenditures are classified by their main purpose. According to your explanation, the first objective of building the sea wall is to prevent coastal erosion from the sea, that is similar to building structures for flood control, for protection against natural hazards. The class for this expenditure is 04.2.1 Agriculture.
Only in case the construction of sea wall served the main purpose of protecting the soil from erosion or other physical degradation induced by human activities (e.g. coastal deforestation), rather than by natural forces, it would belong in COFOG class 05.3.0 Pollution abatement, the category that covers soil protection as part of environmental protection.