Yes, USD will do fine. Even for instance on the buses in Aruba. You need plenty of $1 & $5 bills (for whatever reason $2 bills are viewed with suspicion) because many shops give change in local currency, though its not a problem with stalls & port taxis. Also helps your bartering at market stalls etc. Low denomination bills are difficult to get from the usual suspects in the UK, but you can only ask. But def decline any notes bigger than $20. ..... ............... your ship may be able to help by changing-down (esp in the casino of a US ship) ........ all first cruises are great & the Caribbean is made for cruising, have a good one ...... Chris.