If you stick with the major merchants, then you shouldn't have a problem with traveller's checks. I did have a bit of trouble with traveller's checks at Dunn's River Falls. They only take cash, and it costs 15.00 US/person to get in.
I went with one of the Taxi Drivers who was outside the pier. He was a really nice fellow, when he found out that we only had traveller's checks, he gave us 30.00 cash for my wife and I to get in, came back in an hour to pick us up and we paid him a 50.00 Travellers Check plus a 10.00 tip for the cab fair and the money he lent us.
You can always cash a traveller's check at the purser's desk before you leave the ship or in the casino the night before.
I went to the Bahamas on the 3 day Fascination Cruise - all the banks are closed on Saturday, so we did have to cash them in the purser's desk.
If you do find you have left over checks, you can always use them to pay down your sail and sign account. This is really usefull if you are from a another country, and do not wat to pay the commission to the bank when you are selling. (I am Canadian myself, I have to pay a small premium to buy the US $, then they charge a premium when I convert back to CDN $)