As I read the OP, she was left with no means to pay her bill for expenses run up on the ship, so the purser gave her credit - a loan in effect. In return she was asked to sign an agreement to repay the loan. She hadn't signed this at the point when she asked for her passport, and so they asked her to sign it before returning the passport to her - reasonable precaution I should have thought.

The promissory note was apparently couched in strong legal terms, which seem to have panicked the OP. Perhaps it could have been worded in plainer language, but was no doubt drawn up by the company's (US?) lawyers to be watertight even when signed by a national of another country while in the territory of a third country, or perhaps in international waters.

If you take out a loan from a bank or finance company you usually expect to have to sign a fairly complex and legalistic contract. I can't see that this is any different. Regrettably, the way this was handled seems to have upset the OP, but the purser (who she's admitted was busy and overworked) and seems to have bent over backwards to help her out in an unfortunate situation.