I will take a ferry from Nynäshamn (Sweden) to Gdańsk (Poland), arriving at 13:00. I have a train from Gdańsk at 13:57. According to Google Maps, it's approximately 8 km from the ferry terminal to the railway station. Is there a bus that I can safely take, or will I have to take a taxi to safely catch my train? Are there normally taxis available, or should I order one in advance?
You can check public transport on site http://trojmiasto.jakdojade.pl/?locale=en I made a direct link to your connection; you can change the time, date and places of start and end (by right-click). It's very important wheter your journey will be at weekend or on a workday. At workdays buses and trams runs more often.
Anyway, I think that in your case a taxi is the only reasonable and safe solution, because the amount of time is too little to travel by public transport. It would take about 40 mins and you need time to find your bus stop, find platform on railway station etc.
Obviously you can book the taxi in advance. In Poland such companies are usually called "(Something) Radio Taxi", so you can just Google it searching for Radio Taxi Gdańsk. Examples (with websites in English):
To get from the Gdańsk ferry terminal to the Gdańsk central railway station (Gdańsk Główny) in less than an hour, the only safe way is by taxi. As of November 2012, the ferry from Nynäshamn arrives at 13:00 and the train to Kraków departs at 13:53. For other trains there may be more time; this answer relates to the mentioned times.
There is a taxi queue in front of the main ferry building in Gdańsk. According to the Polferries staff, this is always the case, but they can also order a taxi.
For disembarking the ferry you may have to wait for ferry staff to guide people; at least on the Baltica there is no seperate gangway for passengers, so passengers have to walk on the truck-ramp — therefore guided by staff for safety. It may take a little while (in my case it took 15 minutes) before this occurs. However, the taxi ride doesn't take more than 15 minutes. When I took it the taxi didn't take cards, but instead stopped for me at a ATM, which took another 5 minutes. In any case, I still had 20 minutes spare before the train departed.