For what its worth i have in the past found that water features can be increadably slow to render, then i tried using the external render option (batch render) this did the trick!
This was a while ago. but it really speeded things up for me, so it might be worth a try for a single frame, i dont animate so i dont know if this is possible for an animation, I think water is the most difficult thing to get right in any 3d app.