Terrain hugging over land and sea are two different things.
One can be pre-defined while the second is not and ever changing surface. The critical part will be getting this to between 5 to 10 metres. Then comes another problem.
A missile coming in this low, sees the target comparatively later than a missile which comes in at say 50 mts. Another problem sea skimmers are difficult to be used in rough weather. Else, the guidance and the materials used needs to be really good. As good as a Naval Airforce pilot who has practiced flying by his pants.
More can be learned from
@gambit @Penguin and
@Capt.Popeye @niaz