Royal Caribbean would be a good starting point, 4 star, so good middle ground but could you explain 'favourable exchange rates' to get a clearer response. Just hit the comment button, & type away & I'm sure you'll get some good advice.
Most of the cruise ships operating today are controlled by three major groups Carnival (Costa / Cunard/ P&O / Princess / Carnival), Royal Caribbean (Celebrity / RCI) and Star (Norwegian Cruise Line / Orient /Star. Most are American style cruising with the US$ being the normal currency. Of course on board it is normally a "cash free" society and you can pay at the end in most currencies but normally by credit card.
We dont have a credit card only debit card is this accepted to pay at the end of cruise we are going with princess line.
No cruise lines take Debit Cards. I suggest you get a Credit Card it makes life so much easier. You can pay cash USD at the end of cruise but be prepared to get in a long queue on the last morning. Not a good idea if you have an early flight/transfer etc.