If you book a cruise and don't get drinks, tips or other perks, you leave money on the table.

In order to entice travelers to book a cruise in advance and to choose them over a competitor, many cruise lines run promotions that allow you to pick a perk. When you reserve a cabin, you want to maximize the free stuff you get.

You are likely to see Norwegian Cruise Line's "Free at Sea" promotion, Holland America's seasonal upgrade event and Princess Cruises' "best sale ever" You need to read the fine print to make sure your bonus is worth what you pay.

Sign up for the cruise newsletter for more information.

Free cruise booking perks come with associated fees. Sometimes what you get for free isn't what you were expecting. You could be out more money if you accept the offer.

If you don't have enough reading glasses, you can't look through all that fine print. We list the most common cruise booking perks you will encounter and alert you to what you are actually getting and paying.

Open-bar packages

Be aware of the package you get — because you might not get all the drinks you want. NORWEGIAN CRUISE LINE

There is an open bar on a cruise ship. You don't get all the drinks for free but you do receive a beverage package. It is still a huge perk but be aware of what you are getting.

A free cruise booking perk is offered by Norwegian, which includes a soda package for all passengers under 21. Select spirits, cocktails, wines by the glass, beer, soda and juice are covered by the package. Starbucks, room service, bottled water, and energy drinks are excluded from the list.

Passengers who agree to Norwegian's "free" bar package will have to pay 20% of the package's retail value. That adds up to $21.80 per person per day. For a weeklong cruise, you can get a beverage package for nothing. That is still a good deal for most vacationers. It is possible that it is more than you want to spend on alcohol.

You can sign up for our newsletter.

Cruises have drinks packages that are worth the price. Before buying, you should know what to look for.

Free cruise Wi-Fi

It's one of the cruiser's favorites because of the free onboard internet access. From time to time, lines such as Norwegian and Princess offer free wi-fi to entice travelers. You don't have to worry about paying sky-high satellite connection rates if you keep in touch with family, flaunt your vacation photos on social media, and check your stock.

The bad news is that you can't use free wi-fi. Access may be limited. Norwegian limits its bonus to one device for the first two passengers in a cabin, whereas Princess gives free wi-fi on one device at a time per passenger.

Norwegian only gives a limited number of free minutes, and they don't allow streaming. If a mountain blocks your ship's line of sight with the satellite, no one is going to get any free internet.

How can I get wi-fi on a cruise ship?

Onboard credit

No taxes or tips are involved in the use of onboard credit, which is basically a cruise ship gift certificate. You don't need to worry about hitting the shops.

There are caveat you should consider. The giant "get $800 onboard credit per stateroom" is next to the tiny "up to" next to the advertising. Only sailings of 10 nights or more were eligible for the full amount, with weeklong cruises getting $400 and short sailings getting $200.

If you booked a two- to five-night sailing, you only received $25, not the $50 onboard credit that was advertised.

You can't get the credit back in cash if you don't spend it. It is likely that you will spend less than $50. You might be tempted to use the credit as a discount and buy something more expensive. I used a $50 credit to book a $129 spa treatment that I wasn't planning to treat myself to, until I received the credit. The cruise line's $50 offer convinced me to spend $79 I didn't want to.

There are items on cruise ships that are worth the extra charge.

Free specialty dining

When cruise lines offer a couple of meals on the house, it's a great booking bonus. If you choose the priciest restaurant, you'll get the most value.

Holland America offers a free dinner at either the Canaletto or the Tamarind on six- to nine- night cruises. If you choose the more expensive grill, you will get more value.

Norwegian passengers are charged a 20% gratuity on their complimentary dining perk. A weeklong cruise includes one or two free meals, and a charge of $10 to $17.80 per person in tips for wait staff.

An additional fee will be incurred for additional main courses and premium dishes. This isn't a free meal.

You can have the best meals at sea.

Free shore excursions

DSTEPHENS/GETTY IMAGES

If you don't sail a high-end line, such as Regent Seven Seas Cruises or Viking, you'll have to pay for all your tours in port. A free tour sounds like an amazing money- saver.

That is correct on a line such as Oceania Cruises. Four shore excursions on cruises up to nine days are included in the promotion. Norwegian and Holland America could call their promotions "shore excursion credit" rather than "free shore excursions."

On a weeklong cruise, Holland America gives a shore excursion credit to the first and second passengers. The first person to make a reservation will receive shore excursion credit. The person can't use the credit to book a tour for another person or buy equipment on private islands. You will be paying more to take advantage of this offer as you won't find many tours priced under $50.

There are 10 mistakes to avoid when booking a cruise shore excursion.

Free kids or 3rd and 4th passenger fares

Third and fourth passengers sharing a room and sleeping in bunk beds are bonus revenue for cruise lines. Third and fourth passenger fares are usually lower than the first two passengers and can be used as a booking bonus. It is worth it to comp that extra person, who will spend money on food, drinks and tours on board.

If you want a free adjoining cabin to house your teens with their own bathroom, you have to pay for it. Kids sailing as third and fourth passengers in your room are only eligible for the promo. If you were going to share accommodations, this is still a great deal.

If you are cruising with kids, grandma or your best friends as the third and fourth passengers in your room, you will probably have to pay taxes and port fees.

There are five best cruise lines for families.

Prepaid gratuities

Automatic gratuity charges can add $100-plus to each passenger's cruise fare if you don't know about it. You could ask for the charges to be removed, but I don't want you to do that. It is a bonus when cruise lines give away free pre-paid gratuities and take care of tips, so you can save money.

Tips for up to four passengers in a cabin will be covered in Princess's "best sale ever" The first two passengers are the only ones who will be paid gratuities. It is not out of your pocket if the crew is paid.

You don't need to tip on cruises.

Free cabin upgrade

A Royal Caribbean suite. MICHEL VERDURE/ROYAL CARIBBEAN

Paying for an inside cabin and being bumped to a suite is a dream of everyone. You should keep dreaming. I haven't seen a free upgrade promotion that's so generous. When cruise lines offer free-upgrade promotions, you can get a better room for less.

Cruise cabins can be upgraded from a cheaper balcony cabin to a more expensive one, or from an ocean view cabin to a balcony. You will sometimes see this as a way to get a balcony cabin for the price of an ocean view. A recent Azamara "Double Stateroom upgrade" sale offered a veranda cabin for the price of an inside cabin for the price of an ocean view, but it's not the norm.

You will need to dig into the terms and conditions of the upgrade offer to find out what it will bring you. If you upgrade to a cabin class, you will get a more desirable location as decided by the cruise line, usually on a higher floor.

Sometimes you can choose the location of the upgraded room, other times you can book the lower-priced cabin and the cruise line will move you where it sees fit. Don't accept a free upgrade if you don't want to pick your cabin number.

It's not guaranteed that you'll get an upgrade. You will lose part of your perk package if the higher category rooms sell out.

Bottom line

If certain free cruise booking bonuses are just discounts, they can still save you money.

Time your booking to coincide with a promotion. You should watch out for freebies that will make you pay more than you would have without them. Cruise lines will usually let you opt out of perks if the bonus offer isn't worth it to you.

Are you planning to go on a cruise ship? These are the stories to start with.