Crack on Lower End

You've had that crack for a long time. You just never noticed it.
The visible corrosion blistering under the paint shows the crack pattern.
Salt water, dissimilar metals, heat and vibrations all contributed to the failure.
I'd replace the entire lower unit.

 

that picture doesnt look good, dont think its possible to get a reliable repair with a crack like that, prepare yourself for a new casing at the very least and possibly a whole new l.u. depending on the extent of the wear on other parts, i agree with brett that it looks like its been like that for a while

 

Looks like you're going to have to go price hunting.
Start with your local dealer for the part number and price quote,
then search the web for lower unit rebuilders/exchange
and marine salvage yards. Good luck.

 

you might not be in as bad a shape as it looks... Find an outfit that works on Honda gear and have them evaluate it. The interesting thing will be a pressure test to see if the gearcase (the portion that holds the all important gear oil) is still intact. That will determine whether a repair is possible or even desirable. A quick check you can do before heading to the shop is to crack the lower oil screw enough to allow the oil to seep out a bit. If the fluide looks milky or like a milkshake... your gear case has failed and you're looking at a complete rebuild or replacement. If the oil is good then a repair might be possible. Post up what you find. Lower units are available, both new and used, so a straight replacement is a very good option - if you can afford it....