That's most likely because of the reflections and the lighting boucing in the water, it's alot of calculations to bounce all that lighting/reflections hitting the water and so on. Try without the water and see if it improves, if it does there is not much you can do other than simplifying the scene or waiting.
Edit: You could try adding a spotlight or ghost light to light the darker areas of the scene, because they are pretty much relying on reflection/bounce to be lit, in theory it will take the same time as now to render but you'll see better results in less time. You could also try playing with Path Length iray settings, 7 is the lowest you can get without messing eyes reflections. You can try lowering the Reflectivity of the water. Just testing will really tell you.