..which will be neither cheap nor easy - if indeed sensibly achievable...
SIGH 
We're not talking about turning Apache into a small ship's flight replacement, here. 'Marinisation' for that platform is to allow it to operate from the likes of OCEAN in support of ops ashore etc, and hence to ensure it doesn't get throughly trashed by the environment whilst doing so.
It already
has been achieved; you don't think the trials detachments, over a significant period of time and with a significant number of DLs to the ships, were undertaken with an unmarinised airframe, do you? Just one example: the UK Apaches, uniquely, are designed, and were delivered from the outset, with manual blade fold.
For all that these forums slag off Contractors and DPA, the work done to modify the aircraft and to come up with protective and preventative procedures for the airframe and systems has been undertaken well and effectively. Yes, the ops to the deck are inevitably constrained in some ways in order to respect the capabilities of the aircraft/ship combination, but that's true of your Dark Blue aircraft as well (of whatever Nation).
As for Cobra: Are we the USA? (No) Have we more than one air arm with similar equipment? (No) Could we afford Cobra as well as Apache? (No) So, your point is...?