I search for the information
about EV (enterprise value) of the seaports with
the total throughput of 30-60 million tons (up-to-date or historical data
for the last 2 years). I am not aware of any source making such information
available. The ports themselves are not willing to disclose such data, and
the commercial maritime publishers seem to be not in possession of any
comprehensive information on this subject. I need these data to determine
the value of a seaport and  I would like to use relative valuation models to
determine the true value of the company by comparing  it with companies of
similar size from the same industry.

You are right that ports are not willing to disclose much
information... Some years ago I had similar problem working on a due
dilligence project. Our succeeded to get some date from the customs
offices administration (it was not in USA), and some directly from the
port authority. As far as I remember, it was only the port authority,
the respective Ministry, and the National Statistics office, that had
the information that we needed.

Hope this helps :)