You probably already know this, but as a heads-up to others browsing that site, quoted prices for cruise ships are per-person based on double occupancy, so you'll only get a rate as low as $50/day/person if you're traveling as a pair sharing a room. If you're traveling single, you pay anywhere from a 50-100% supplement. Picking a few of the ones from that site at random, it looks like most are charging the full double price if you want to book a room as a single, so ~$80-100/day. Add in the ~$10-15/day quasi-mandatory gratuities and you end up not saving much vs. the $120/day container ship, though certainly for a much different kind of trip.
Heads-up: English is not the primary language of most passengers, but from what I've read, the crew can accommodate it.
Norwegian also has some that are under $100/day including taxes and gratuities. The one below is $968.95 for 13 days ($75/day), assuming you take the promo where the cruise line pays for the gratuities.
