There are two seals in the upper case that seal the vertical driveshaft. The upper is larger and keeps the oil in while the lower is smaller and keeps water out. Was the drive pressure tested and failed?
If the leak is one of these seals it's a pretty easy fix but you will need a few special tools.
An upper retainer wrench to remove the drive gear assembly and driveshaft. Once the driveshaft assembly is out, remove the top cap and reach in and remove the drive gear assembly. This will reveal the two upper vert DS oil seals. Remove seals and install new ones. Merc makes a special tool for this and it works wonders. I've seen it done the garage way with a socket but to each their own. Reassemble, pressure test and go boating.
This is a generic rundown but should give you an idea of what the process will involve.
If we were on the same coast I would do it for you for free. I've got all the tools and am pretty good rebuilding drives.