I have never been on Fred Olsen Cruises so I can only comment on other Cruises but I think they are much the same,nearer the time of departure you go online to register as this saves time when you are at the dock, you are asked to give a credit card details and you must take this credit card with you on board ship as they will check it with you, you will be given a seas pass just like a credit card and you must use this for going on and off the ship,this is also used when you make any purchases on board,eg buying from the shops, buying drink, this is you identification and also the key to your cabin
I would take U S dollars these can be used as Gratuities to the Cabin Steward/ Waiters if you have not already decided to pay with your onboard account
I hope this has answered some of your questions have a lovely I am sure it will be brilliant .
The credit covers all your costs you purchase on the ship you are now in the cashless society except for the casino where you would need dollars or you can charge it to your cabin account, not a good idea as you can quickly run up a big bill unless you are winning. In the rest of the Caribbean they will take US dollars. We usually take US travellers cheques and cash them into currency at the bank on the ship or of course ashore you can also use your credit card. As we take the TC's with us we have already got the best rate back home as the ship will not be as good.
Agree that the US Dollar is the best currency although Euros are widely accepted. In many places throughout the Caribbean credit cards are widely accepted and the exchange rate on these is usually excellent. Remember that on Fred. Olsen ships tips are paid in cash so I always take sterling travellers cheques to cover this cost. You will see that the recommended level of gratuity is £4 per day per passenger, £2 for the cabin stewardess and £2 for the restaurant waiters.I have found that on my many cruises with this line that these are well earned. I assume that you are on the Dover/Caribbean/Dover cruise departing on the 19 October. So am I. I hope we have a great cruise.