The Amphibious Hook is a type of theoretical maneuver that allows for a naval support of a land offensive. It is either an operational or tactical offensive, with the Heavy OPFOR Operational noting that such ones would never be done outside of extensive air support. The document also argues that it generally would take the form of an amphibious regiment/brigade in the first wave and then normal mechanized troops unloading on the shore after the beach was cleared to continue the push. But of course, depending on shipping, it could easily be more.
(Brief note: Strategic amphibious operations are D-Day and even Inchon. Tactical ones are things like doing a boat raid. Operational ones are, fitting that level, more vague and mean things like ‘land a big enough force to divert their reserves so that the main land push can run more freely’).
The section on amphibious landings (Heavy OPFOR Operational sec. 2-13 to 2-15) also speaks of naval units being an easy way to reinforce airborne ones, assuming the geography works. There’s also, as happened in the Gulf War, the threat of an amphibious hook.
Ironically, one of the best ways for a defender to counter an amphibious hook is to ignore it. Or if not ignore it, recognize that it’s going to have trouble moving inland and can be contained with second-line forces and not divert too much to stop it, leaving the opponent with a small toehold always at risk of being cut off.