I would certain get some US dollars before going and would use Marks and Spencers or The Post Office for the best rates. I would also recommend using a Nationwide credit card as their rates are always better than the tourist rate with no commission and always better than the ship's rate. If you take Travellers cheques take them in US dollars ... Nei.
We always get our money from the Post Office before we go and have a Nationwide credit card that we just use for holidays abroad...as Neil says they don't charge commission...if you find you don't have enough money or if you need a bit of local currency (other than dollars) use your card in ATM...travellers cheques are not used much these days we have found.
While on our last cruise my wife and I won around $700 in the casino. We decided to clear our cabin spend in cash. The rate on the ship was very poor compared to back home. I have since opened up a Nationwide account for my future cruises.
Is the onboard account in dollars or pounds on Ventura in the Caribbean?? Does anyone know?
Why buy Travellers Cheques when a hole in the wall is probably as good a deal, if not better & don't use the ATM on the ship if they have one.