The best exchange rates can normally be found via banks and the high street, so in this case you may find it is better to get your money before you set off.
However, you can exchange money onboard most ferries and whilst you are abroad. You will also be able to use credit cards in many major transactions. We would recommend doing research into exchange rates and charges however.
If you are planning on using your card abroad, please make sure you notify your bank, as many cards will be blocked by the bank if unfamiliar transactions appear. This can leave you stranded, and it is always advisable to have enough cash on you if this occurred. For example, you should always have another method of paying for fuel if your card failed.
If you are planning to travel abroad frequently you may wish to stock up on currency for future trips when exchange rates are favourable.