lkm, that is too slow.  this is the part where it gets complex:

• a posersurface for water may have 0 diffuse, meaning no IDL bounces
• a posersurface for water may have some reflection channel or node(s), which may only need 2 RT bounces
• a posersurface for water may be expected to have indirect specular fx of refraction and reflection, which aren't supported in initial release
• a posersurface for water may have substantial displacement, which may also slow down IDL

what I'm guessing is to try rendering the water only, with no IDL, then render the rest (no water) with IDL and then composite (unless already attempted).  please post screen shot of render settings (and render script if desired, which is largely redundant with render settings).