the best way for a Polish national to find training
The only way I see now to get trained here in Poland (beside attending to a Maritime University) is to join a stationary training center. Check this site about possible marine courses and prices.
If you want omit this step, try to contact with Polish Steamship Company (Polsteam) - at present the greatest ship operator in Poland - about training/working possibilities over there.
The another big shipping company in Poland are the Polish Ocean Lines: en.wikipedia.org/wiki/Polish_Ocean_Lines