Most people don't like to complain and most people don't like complainers but there are times when something goes wrong with you travel plans that necessitate an adjustment. Customer service is a tough job especially if it is your only job. So put yourself in their shoes for a moment when you need to complain about something and serve your complaint as a sandwich. Yes, a sandwich.
Start off with a compliment (the bottom layer or piece of bread). Think of something the travel agent, cruise company, airline, etc. has done well in the past or of someone who has done a nice job there and tell them about it. Or at the very least start off with "Hello, how are you today?"
Ease into your complaint. Make sure you have all your facts straight and documentation in order (that's the meat of your sandwich) so that you can check it against the response from customer service. Explain as simply and unemotionally (meaning no yelling and screaming) as you can but be firm in expressing what your expectation was of the original booking.
Add some condiments in the form of your confidence that the situation can be worked out since you are sure the company wants you to be happy with your choice of them.
Then top off the conversation (top layer of bread) whether the matter has been solved immediately or not
with a thank you and another compliment. If they need to respond further they will be much happier if you leave them with a nice "sandwich" to chew on.