Cruise ships are exceptionally good at finding things to sell you. Cocktails, massages, photographs of you looking startled in a restaurant. Even the prospect of sitting somewhere slightly nicer can come with a price tag.

But what happens if you refuse to buy any of it? Could you spend a week at sea, eat three decent meals a day, see the sights, enjoy the entertainment and disembark with a final onboard bill of precisely £0?

Surprisingly, yes. Although it requires a little planning, an understanding of what's included in your fare and, depending on the cruise line, a potentially awkward conversation about gratuities.

Here's how the experiment would work.

Can you eat for free throughout a cruise?

Our running onboard bill: £0

Food is the easy part. On mainstream cruise lines such as Royal Caribbean and Princess Cruises, your fare covers meals in the main dining room and buffet, with additional complimentary venues depending on the ship.

On Royal Caribbean, for example, you could start with breakfast at the Windjammer, return for lunch and have a three-course dinner in the main dining room, all without adding a penny to your account. You could also eat considerably more than three meals, although at some point the experiment may become a medical concern.

The trick is knowing which restaurants are included. Speciality dining generally costs extra, and room service can attract delivery charges. Princess Standard, meanwhile, includes main dining and buffet meals but charges separately for several dining upgrades.

There's no requirement to pay for a premium restaurant simply because you're onboard for seven nights. You could enjoy a different menu every evening in the included dining room and never feel particularly deprived.

What can you drink without buying a drinks package?

Running onboard bill: £0

This is where the challenge starts to require a little restraint.

Royal Caribbean includes water, regular coffee, tea, lemonade, iced tea and selected juices at its complimentary dining venues. What you generally won't get for free is alcohol, speciality coffee, bottled water or fizzy drinks.

So yes, you can stay hydrated, enjoy your morning coffee and have something other than water with lunch. You just won't be ordering a poolside Aperol spritz unless you abandon the experiment.

Incidentally, a drinks package isn't necessarily the economical choice even if you're not attempting to spend nothing. The amount you need to drink to make some packages worthwhile can turn a relaxing holiday into a rather demanding administrative exercise.

Are cruise ship entertainment and gyms free?

Running onboard bill: £0

Here's where cruise holidays become surprisingly generous.

Princess Standard includes access to pools, the fitness centre, live entertainment and theatre shows. Royal Caribbean similarly includes a wide range of performances, activities and sports facilities, although certain attractions, classes and premium experiences may carry a charge.

In practical terms, you could spend the morning swimming, use the gym, attend an included quiz, watch a live performance and finish the evening listening to music, all without buying anything.

You'd need to avoid the spa, casino, paid fitness classes and certain bookable attractions. But considering how much is already available, spending nothing wouldn't necessarily mean spending the week staring at the horizon.

Although that's free too, should the mood take you.

Can you explore cruise ports without paying for excursions?

Running onboard bill: £0, provided you explore independently

A cruise-line shore excursion can be one of the largest optional expenses of a sailing. Fortunately, you don't have to book one.

Some ports are wonderfully straightforward. In places such as Dubrovnik's Gruž harbour or Barcelona, public transport offers an alternative to organised tours, while smaller ports may allow you to walk directly into town. There are also destinations where simply wandering the streets, visiting a beach or exploring a waterfront is a perfectly satisfying way to spend the day.

But here's an important distinction: spending nothing through the cruise line doesn't necessarily mean spending nothing ashore.

At some ports, the ship docks in an industrial area where walking is prohibited or impractical. A shuttle, taxi or local bus may be necessary, and the cruise line's transport isn't always complimentary.

You could theoretically stay onboard at every port, which would preserve a perfect record of spending nothing, but seems a slightly peculiar response to having travelled hundreds of miles to get there.

For a genuinely low-cost cruise, choose itineraries with walkable ports and investigate local transport before booking. The savings are often substantial, even if your daily expenditure doesn't remain at absolute zero.

Can you avoid paying for cruise ship Wi-Fi?

Running onboard bill: £0

Yes, provided you're comfortable being largely unreachable at sea. Depending on your disposition, this may be the greatest luxury of the holiday.

Download your books, music, maps and entertainment before leaving home, then use Wi-Fi ashore when it's available. You can also arrange with friends or family to check in whenever you reach port.

You don't necessarily need paid internet access to use the ship's own app, either. Royal Caribbean allows passengers to connect to its onboard guest network and access the Royal Caribbean app without purchasing an internet package.

Be careful with mobile data, though. Ofcom warns that phones on cruises can connect to maritime satellite networks, where charges may be considerably higher than ordinary roaming. Airplane mode, with Wi-Fi enabled when needed, is the sensible default.

The catch: cruise gratuities and service charges

Running onboard bill: potentially no longer £0

We've reached the part of the experiment where the arithmetic becomes less cheerful.

Many cruise lines automatically apply daily gratuities or service charges unless they've already been covered in your booking.

Royal Caribbean, for example, currently charges US$18.50 per person, per day for most cabins, rising to US$21 for suites. For a couple sharing a standard cabin on a seven-night sailing, that's US$259 in gratuities.

Princess likewise automatically adds crew appreciation to standard-fare accounts unless it's prepaid or included in a package.

Royal Caribbean permits passengers to adjust gratuities at Guest Services, but asking to remove the crew's compensation simply to achieve a pleasingly empty bill rather misses the spirit of the experiment.

The better solution is to choose a fare that already covers service charges, or pay them before travelling. MSC Cruises, for example, treats its hotel service charge as a mandatory part of the total holiday price, generally collected at booking rather than added onboard.

That gives us the crucial distinction: you can finish with no new onboard charges without having paid nothing beyond the advertised headline fare.

Which cruise lines make spending nothing easiest?

Some operators offer a considerably easier route to a zero onboard bill.

Virgin Voyages includes dining, basic Wi-Fi, soft drinks, fitness classes and entertainment within its current fare proposition. However, gratuities are now itemised separately for newer bookings, with the option to prepay them or settle onboard. A £0 final bill is achievable, but you need to account for that charge when booking.

Princess Standard provides enough complimentary food, entertainment and facilities for a very enjoyable holiday, although drinks, Wi-Fi and crew appreciation require more consideration. Princess Plus and Premier bundle additional amenities and gratuities into an upfront package price.

Royal Caribbean also makes the experiment entirely plausible, provided you stick to included restaurants and drinks, avoid premium activities and arrange gratuities in advance.

The best choice ultimately depends on what you'll use. Paying extra for an inclusive package you barely touch is no great financial victory, even if the final bill looks immaculate.

So, can you finish a cruise with a £0 onboard bill?

Yes. And the most surprising finding is how little you'd necessarily have to sacrifice.

On a seven-night mainstream cruise, you could have every meal in an included restaurant, drink complimentary beverages, use the gym and pools, enjoy the shows, explore suitable ports independently and spend a week without paid internet. Your optional onboard purchases could genuinely amount to nothing.

The difficult part is making sure the unavoidable or automatically applied charges have been dealt with before departure. Gratuities, service charges and any other compulsory costs need to be included in your calculations, not conveniently forgotten.

Of course, the experiment also requires you to walk past the cocktail bar seven evenings in a row. For some people, that may prove a more formidable challenge than any financial constraint.

But there's something rather satisfying about discovering that, once you've paid for a cruise, an excellent holiday doesn't necessarily require you to keep paying for it.