This is very strange...i have a Yamaha 115hp 2 stroke which is 15 years old, and is as good as new. And it spends 4 to 5 months constantly in the sea. Something must be definitely wrong with yours Icom.
@ aidanxuereb its actually very common on yamahas that the the tilt motor cover rusts away like crazy. Do a google search and you will find out how common the problem is. Many in the US when replacing their tilt motors, they buy after market cause Yamaha tilt motors apart from being very expensive are know to use inferior metal for the motor cover. In your case you probably have a particular model which didnt have this problem.