Some people get decent results with just a single
through-hull connected as the R.F. ground, and I think that using the saildrive
as a ground will probably do a pretty good job. You should review the connections to all your underwater metal, to make sure that you are protected against electrolysis
(and other electrical
Definitely look at the discussions here to see what antenna
systems other people are using, and why. It isn't really voodoo, but some times it might as well be because of all the variables. You can probably put together a better ground system, but remember that whatever you do will be a compromise of some sort. Some times "good enough is perfect", if it gets you where you need to go.
Be sure to test it ahead of time on all frequencies you want to use.
-Paul - wb6cxc