There is no automatic right to cancel any contract, just because you change your mind, even if it is within 24 hours. There is one for airline reservations. I would certainly dispute the cancellation charge, if it was nowhere in the documents. However, I find that odd, if there was not a cancellation charge in the contract, as otherwise, people could make a booking and cancel the day before sailing with no penalty. 3.5% sounds like a credit card fee to me..