Currency used on Viking Line from Stockholm to Helsinki

You can use SEK, EUR, or pay by credit card.

Thank you Laura.

we did a Viking cruise a few years ago; it was a river not an ocean cruise. At that time all the drinks we bought at the bar were charged to our room and were paid for by credit card at the end of the cruise.

See:

http://www.vikingcruises.com/oceans/frequently-asked-questions.html

"How will payments onboard be handled?"

Edit: For that other company, whose entire business model is based on doing business with as many currencies as possible, something like a Duty-Free shop at an airport:

http://www.sales.vikingline.com/en/my-booking/good-to-know/means-of-payment/

I'm pretty sure that The OP is referring to a ferry run by this company:
http://www.vikingline.com/en/
rather than the company that runs the longer cruises.

Laura,
Thank you for clarifying. Yes, I was asking for the Viking Line that runs overnight from Stockholm to Helsinki. Still really appreciate for all your help.

It never occurred to me until this thread, that there are two different companies called Viking that run boats in Europe! Yes, the river cruise company is not the same one as the the one that runs boats from Stockholm to Helsinki.