Are you hearing any noise from behind the engine, like a growling, or knocking when you turn or tilt the drive up? That's a sign of a leaky driveshaft bellows and corrosion damaged gimble bearing and u joints. As normal maintenance you should:
have the drive pulled at the end of each season
inspect the driveshaft bellows for cracks, water inside, milky grease, all those suggest that the bellows leaked and has to be replaced as do the gimble bearing and u joints.
if the bellows is dry inside, check for cracks, check for smooth operation of the gimble bearing and u joints, grease them if grease-able, lube the driveshaft splines with OMC/Evinrude triple guard grease when installing.
Just doing this, with outdrive oil changes, will extend the life of your drivetrain components greatly.
I have had the Cobra/SX style bellows last as long as 10 years if you store the boat with the drive down. If you have to store it with the drive tilted up, regularly check for cracks and replace as soon as small cracks appear, that's a sign that its time.
The other causes, cooling hoses, etc are things you should check at the start of each season.
Eventually corrosion starts at the point where the exhaust Y pipe is bolted to the transom mount, this will cause a leak at the very bottom of the inner transom area and repairing this requires pulling the drive and engine.
The worst kind of leak is caused by a rotted transom which is starting to flex and then the transom seal no long can seal out water. This means the boat needs major structural work and everything (drive, engine, inner and outer transom mounts) has to come out and the transom replaced.
The fact is that I/Os have many places that they can leak water in, this is why after having one a long time many people switch to outboards to get away from these headaches.
