Japanese Mariner / Yamaha Lower Unit Won't Come Off

[amundsen1911]

I have a 1993 Japanese Mariner 15hp built by Yamaha. The engine has stopped pumping water so i had planned to remove the lower unit and replace the impeller. Having removed the four bolts and disconnected the shift rod the lower unit won't drop. I have checked everywhere for hidden bolts including under the zinc anode and can't see any. Gentle persuasion with a rubber mallet has also had no effect. Any help with this would be greatly appreciated.