Can I use my Schengen visa for different purpose other than I applied for?

There are many special rules for seamen (e.g. around stamping and entering without a visa) and a lot of dubious information on the web so it's difficult to know for sure. In principle, a multiple-entry type C visa can however be used for any purpose. That's in the nature of such a visa, detailed at length in the Schengen visa code and associated handbook published by the EU commission.

There are no provisions in the key Schengen regulations for a separate category of transit or seafarer visa and no way to easily distinguish your visa from any other Schengen visa (“BNL2” is a technical code and does not signal any restriction nor is it specific to seamen, see What does BNL2 Schengen Visa mean? I have one from Netherlands. Can I travel to Spain now? and Meaning of “BNL1” and “BNL2” on Schengen visa sticker?). Consulates also refuse to entertain Schengen visa application when you already hold a valid Schengen visa. All this leads me to believe a visit to Poland should be allowed but I have no first or second-hand experience with this exact scenario.

One step you could take to preempt issues at the border would be to email the relevant Polish consulate or even the ministry for foreign affairs (if you can find an email address…) It's not always helpful and legally irrelevant but if they do give you an answer suggesting it's OK, you will have some proof you were doing your best to follow the rules and are not trying to be deceptive in any way (that's why you will want to do this by email, not by phone). There is however a small risk they would tell you you cannot do it, ask you to apply for a national visa or give you some confusing advice.