The water passage
in a saildrive
is mostly wide open leg and does not need to be descaled at all. Water fills the whole leg through vents in the side of the drive and a hole on the bottom. Water is brought inside the boat
through a valve threaded into a short intake passage
. Any scale that could form at all would just be in that passage the valve threads into. For that, you can simply open or remove the valve, stick a bottle brush into it and move it up and down. However, I doubt you will find any scale in it worth taking the trouble for.
I have never seen manufacturer recommendations for descaling, so I think this is a worry you should just throw away to make room for other more important worries - boats require a lot of worry, so you need to prioritize...