I've cruised 4 times with carnival on 3 different ships and NEVER have I been charged a Port tax in addition to what I paid for the cruise. The reference may be from many cruise web sites that advertise low fares and when you investigate find they don't include the port taxes. But if you have paid up and have a booking, there is nothing extra for port tax. The daily gratuity will be added to your sail and sign card when you board however.
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