I thought a little about this Hannes. Not sure if this would work for your animation but for HD (full
)
You could render the water for example in half HD and the ship in HD in separate layers?
And-or maybe rendering only half the frames (even frames only) the water part and the ship full frames etc.
With motion blur it could work maybe. Depending of the camera move-speed.
From one of my old animations i rendered the background half HD and only the foreground in HD.
It had kind of the full quality HD feeling.
But it looks like you are already rendering.