The upgraded and new build Twin Hueys the USMC operates are marinized, so it's definitely doable.I would dare to add to that:
Range: minimum 5000 nm
Speed min. 18 kts. That is 20% above MCDV's 15 kts (wikip.).
Main Gun: minimum 40mm, best 57mm (anti-aerial role)
Propulsion: electric motor for at least up to 12-15 kts.
I think there is no agreement yet on wether hangar is required or not. IMO small-medium helicopters (3 to 6 tons) would be very useful. Bell 429 or 412 (griffon), as someone has already mentioned, would be nice candidates.
Is there a marinized version of them?, i mean, foldable rotor, corrosion-resistant, strenghtened landing gear and so on.