Here is the scenario:
Two factions, one defending its planet, the other attacking it. The winning condition for attackers is capture of leadership, so boot on the ground are required. The planet is Earth-like, the capital metropolis is coastal. WMDs on the surface are off the table.
Defenders have a wide range of equipment at their disposal, from surface-to-orbit missiles, to spaceships, satellites, planes, VTOLs, airships, choppers, tanks, boats and submarines.
Attackers are capable to launch and recover planes from orbit. They can launch troop transports from orbit as well. They have no lack of orbit-to-surface artillery either. They can match defenders in space, in the air and on the ground, but not really on the sea.
Attackers can rain death from above on anything on the surface, that includes boats. While boats can be used as a platform to launch aircrafts, troops and artillery strikes, those capabilities are redundant. Bringing in boats from space is somewhat of a logistical problem.
On the other hand, opening another front could have its advantages to divide defenders' attention. Launching planes/troops from the sea surface might also be less risky. It would be a new tactic, so there is the element of surprise to consider.
The first question would be: Is it even worth taking the fight to the sea?
Now, assuming we want to fight aquatically, attackers have developed a new type of vehicle, dubbed Space/Naval Hybrid Assault Platform (military loves acronyms). It is a space-worthy ship specifically designed for the task of landing on water and floating thereafter. It cannot takeoff without assistance, and cannot fly in atmosphere beyond controlling its descent.
It could conceivably be single-use, e.g. you could disassemble it for building materials for a base of operations on land, or you could just beach it for similar results, or it may just be a fancier carrier/assault boat.
So the other question would be: What are benefits and drawbacks of such ships, and what would they best be used for?