I have had the same problems with my 4.3 Merc dying as it went to reverse. I used the diagram in Don S's response above - look for how to adjust the shift cable (http://www.mercstuff.com/howdoi.htm) and it was a piece of cake. Thanks Don - ever the Man! Tested her on the muffs and brought the idle down to about 750 and she shifts like a charm. Out on the lake tomorrow to give her a real test (and then teach some worms to swim...)Yep, classic shift cable problem
Here are a couple links to help you fix it. First one is the online OEM manual.
http://www.boatfix.com/merc/Servmanl/6/6covr3.pdf
Something to help sort things out a little easier.
http://www.mercstuff.com/howdoi.htm
Tested her on the muffs and brought the idle down to about 750 and she shifts like a charm.