@squaz05 agreed. Although, if you're on xbox and want to play SoT like it's CoD on boats, buy yourself a M+K for xbox for the most authentic FPS experience.
Rare and MS gave everyone options. If you specialise in the player vs player PvP M+K is best, not required, but best. If you focus on ship v ship, controller is best, again not required, but best.
If you just want to veg in a comfortable chair and see where the sea takes you, controller is best. You might need to deal with sweaty M+K pirates, but you'll be comfortably seated, which I personally find helps keep me in a relaxed state of mind about it (theoretitcally, depending mainly on which of my many multiple personalities is at the helm at the time).
There are plenty of options for how you approach the game, the choice is yours. This is why I'm not against an opt-out in principle, it's just another option. I think it's important that the implimentation of this option is handled in such a way that it doesn't limit the options that are already available.