Was the original idea to implement a bonus for the whole ocean or only the alliance possessing the fort? The first option wouldn't make sense since no one would fight for it...but I just noticed that it leads to another interesting idea I might mention in the next time, once I checked whether it already exists.
As bad as I feel about quoting my own post I have just changed my mind since I have noticed that the idea I had in the very moment I posted the sentences above is closely related to the oceanic fort idea. I was thinking of a general ocean bonus also created by a building in the middle of the ocean. The difference would be, that it wouldn't matter who's holding the fort/building - the bonus would affect everybody in this ocean. There would be a number of different buildings which could provide different bonuses. (e.g. attack bonus, defensive bonus, resource bonus).
One could also say, that the player holding the ocean building has the ability to switch this effect on and off (maybe with a cooldown of several hours) (this could make the ocean building important in ocean internal wars). Since the whole ocean is depending on that bonus everybody should be able to contribute resources in order to expand the ocean building. An immense sum would be needed for the next level. Attacking alliances from other oceans could now try to capture the ocean building in order to switch off the bonus.
Since the bonus would affect the whole ocean one could even decide to increase the bonus given by the building.
Cons:
It could potentially be abused by people who pretend to protect the ocean building while actually waiting for an opportunity to switch off the bonus so that their allies from outside can strike.
I'm not quite sure but maybe this would be a change which is too massive. It could interfere with the ideas the developers of this game had. In that case one should think about the size of the bonus.
Summary: It's pretty much the same as the ocean fort but with more types of forts, an ocean wide effect and the ability to switch it off.
I'd appreciate feedback especially when it's showing me which parts I haven't thought through.