After 42 Royal Caribbean cruises, I still make 1 huge mistake

You can make a lot of financial mistakes while on a cruise, and you may regret some more than others.

On some occasions, a bad choice turns out to be a positive.

That happened to me a few months ago on Celebrity Reflection, where after imbibing a few drinks, I purchased a Macallan liquor-tasting class for the next day. My experience with classes like that on other cruise lines is that they are really just sales seminars.

In this case, the $60 class was a wonderful education in discovering how Scotch gets made and how you pick the ones that fit your palate.

I would have taken another class with that bartender-teacher when I sailed on Equinox again a few weeks later, but that sailing offered only the one I had already taken.

I've also made mildly dumb mistakes, like not booking a dining package and then deciding I wanted to eat in the extra-fee restaurants. Buying meals a la carte costs more than buying a package beforehand. And of course there were times when I paid more for the internet or a drink package because I did not reserve it in advance.

Those are rookie mistakes but generally low-dollar affairs. On my most recent cruise on Royal Caribbean's (RCL) Celebrity Cruises, however, I made a blunder that cost me nearly $200. And to make matters worse, I had done the same thing on the same ship just a few weeks before.

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Make sure you put your phone on airplane mode when you leave port. Image source: Royal Caribbean

Airplane mode is your cruise friend   

When you get on a cruise ship, it's imperative to put your phone in airplane mode. This prevents your phone, even if you're not actively using it, from roaming onto networks that charge you extra money.

When T-Mobile  (TMUS)  sees that you are not on U.S. soil, it sends you a text message to warn you about these charges. That's usually enough for me to avoid these extra costs, but it's really easy to make a mistake that can cost you a lot of money.

Cruise ships leave the U.S., sail in international waters and make stops in foreign countries. Some of those countries, including Mexico and the Bahamas, are covered by my T-Mobile phone plan, so when I'm in those places, I leave airplane mode.

Again, when you do this, T-Mobile sends you a message about how much data you get and whether there are costs associated with calls or texts. In some countries, for example, texts are free but calls cost 50 cents a minute.

In some cases, I have made calls knowing that there are costs associated, even long ones because personal or work-related reasons required me to do so. My two big bills, however, were not intentional. In both cases, I am to blame.

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Do not do this on a cruise ship

On my two most-recent cruises, both on Celebrity Reflection, I barely left the ship. I spent most of my time at Cafe al Bacio, the on-board coffee shop, with my laptop in front of me.

When I was in Bimini and Key West, I switched my phone out of airplane mode so I did not have to keep switching my internet back and forth from my laptop to my phone. Doing this is a slow process since you must pull up the login screen, type in your credentials, and choose to switch devices.

In theory, there's no harm in turning off airplane mode when you're in a port that offers service from your carrier. The problem arose when we left and I neglected to turn it on again. That left my phone running up a bill: It stayed in my pocket all night while I watched live music, gambled in the casino and talked with other passengers.

The charges continued to accrue as I plugged in my phone to charge it and then went to sleep. I didn't even think much of it the next day when I saw the text message warning me.

In fact, I realized my mistake a few days after my cruise — when T-Mobile sent me a note alerting me to nearly $200 in foreign phone charges on my bill. It was an easy mistake to make, given that I made it again a few weeks later.

My best advice: Put your phone in airplane mode unless you leave the ship. And if you do leave airplane mode, put a note in your cabin to turn it back on when you get back on the ship.