The simple answer is that you will need european cover for a mediterranean cruise....if you are having additional holidays within the year which may be outside europe it will be worth considering an annual worldwide policy. try travelcover.co.uk who are good valu.
I take it that Thundersley is the one in Essex and that you hold a British passport as it could be different if this is not the case. Assuming that, there are four main types of insurance for holidays or cruising. The first two being cover for the holiday itself for the period of the holiday and covering you up to that date for cancellation. This could be Worldwide or Europe cover. The next two would be similar except that it would cover you for the whole year for Worldwide or Europe cover. You must choose because if you intend to have another holiday within one year then the latter might be better and then decide whether you a likely to go outside of Europe. If this is to be your only trip then a Europe cover insurance will suffice for this cruise you are proposing. There is another kind which is good if you get the correct cover as with, say Lloyds TSB platinum debit (and I think credit card), whereby you can have permanent cover until you are 80 for free. With this option please make sure that it is covering all the aspects of holiday insurance and not, like some, offering only "Accident Insurance" which is of little use...Hope that helps...Nei.
Halifax and bank of Scotland also provide free annual worldwide travel insuramce as part of teir Ultimate current account. However if you have any pre-existing medical conditions you have to phone them first and they will tell you whether or not you have to pay an excess to be covered for the condition or wher=ther you dont want to be covered for it. Always read the small print.