Dynamic currency conversion (DCC) is where the merchant (in this case, Marriot) "offers" to charge your credit card in your home currency. Generally, the merchant's bank charges more for this than your home bank would charge, and the merchant's bank shares this profit with the merchant.
According to Visa, this is something that's allows to be offered optionally, but cannot be forced - and they merchant must ask.
Sadly, my experiences with Marriott in the UK is that they strive at every opportunity to force this upon you.
One example is on the advance purchase rooms. There seems to be no option - and this is in direct disagreement with what I've been told by Visa.
Has anyone else noticed they are charged in their home currency, either without being asked, or even when they have said no?