Definition
A saildrive diaphragm is the flexible rubber seal between the saildrive leg and the hull, keeping water out of the vessel; it is a safety-critical, time-limited component replaced on an interval set by the drive manufacturer.
This is not an ordinary gasket. It is the seal between the sea and the inside of the boat, it is made of rubber, and rubber ages whether the boat is used or not.
Why the interval is not transferable
Every drive maker sets its own replacement interval and its own inspection requirements for its own design. An interval quoted for one manufacturer's drive tells you nothing about another's, and applying the wrong figure is how boats end up with a seal years past its life. Use the schedule for your exact drive, from the current documentation.
Many installations also use a double diaphragm with a warning arrangement between the layers. Understand which you have and what the warning means.
When buying a boat
Ask for dated evidence: when it was replaced, by whom, and what else was done at the same time. Absence of evidence should be priced as though the work is due, because it is. See how to assess a used marine engine.