Is there a default SSH password on Mac OS X? If I create an account, and don't give it a password, what's the password to SSH it?
Having no password on the OS level effectively means that ssh is disabled for that user. The protocol insists on a non-zero password and thus the hashes of whatever password is supplied fails to match the absence of a hash.

You must supply ssh keys or otherwise arrange some authentication to log in should your account have no password.