It is probably at the bottom of the ocean, the battery is too heavy to wash ashore, it is like a brick.
All you can do at this point is talk to DJI before buying a new bird, they may be able to offer some kind of assistance. I followed a thread about a similar situation and the guy was patient talking to DJI, finally they decided to replace the bird.
Like you mentioned in your post is that if there is a bright side about this unfortunate event is that you were flying in water.
I am sorry for your loss.!! It can happen to any of us.