I cannot take credit for this idea. I read it on the Internet somewhere and can't find it now for the life of me so I thought I would post it here in case someone had the same problem.
I carried my boat to a recommended local mechanic to have the impellers changed on my twin 2001 Mercury 250 EFIs. At just under 400 hours both water pumps were going strong. Problem is , The lower units had never been off in 19 years of salt water use.( I know, I'm an idiot. I knew it would be difficult. Thought the mechanic might know some tricks I didn't ) Two days later the mechanic called me and said "sorry, I tried, but these lower units are not coming off. When one of the impellers goes out, these engines are done." Not ready to give up I found this trick online: loosen all of the lower unit bolts approximately 1/8 of an inch.( mine were the big Nylok nuts, so there was no danger of the lower units falling off ) put the boat in the water and run it for about 20 minutes going from on plane to idle a few times. At idle shift from forward and back to reverse several times. When I returned to my dock, both lower units had separated. With the boat back on the trailer and all bolts, nuts removed I shook the starboard lower unit once and it almost fell on my feet. The port came off just as easy. Changed both impellers and reassembled with grease/anti-seize compound. I've learned my lesson!
Hope this can help someone else. Saved my rear!