On my boat there are zinc anodes held on by embedded bolts,
one has lost it's nut. How can I identify the nut needed
from only the thread?

I can measure the diameter, and can measure the number of
turns over a set distance. Is there a site I can look up
stuff like this?

Removing the embedded bolt and bedding a new one is well
beyond a job I'm happy to tackle.

Of course, this may all be moot, I may find, when the
boat's lifted, that the bolt has been eaten away because
there was no anode on it (I hope others in the area have
compensated).

Thank you for any advice you can give.

Justin.