That sounds pretty awesome, I've always liked games that have a lot of variety in combat and war mechanics, just because I love creative tactics and strategies.
Though, I dont know about that AI sea monster appearing in middle of the PvP battle. Sure, it would add some twist to the fight, but it could pretty much wreck the whole balance of the naval enviroment, would you like to lose a long battle you fought so hard and were on winning, just to find out that some giant crab is eating your ships for lunch? I think I wouldn't enjoy that so much. Probably you could make it optional for yourself to have AI to involve in your naval warfare?
Everything else sounds fantastic to me, hunting sea monsters, exploring seas and islands will make the game more interesting, one question about that though, I know that there will be big world to play in, but its sure that some players want to play on only ground and some focus on naval? Is it possible to focus your armies on huge navies or something like that, I dont find it possible for everyone to be close enough to the sea to play effective naval, since they are too far away from shore?