It could be a rusty bolt head, but the white foamy material seems to
suggest there's more going on here.

There is a type of "electrolysis" that can occur in wood, particulary
when wet.

It isn't unusual to find this in a boat, and the areas around the
rudder posts, shaft log, through hull fittings, etc, are prime
offenders. The process initially removes the soft, pithy portions of
the wood.

If it hasn't advanced too far, the process can be arrested with dilued
vinegar. Get an on-site professional to render an opinion about the
severity of the problem.