Can you make an example what you think the problem is which has to be solved or explained? I understood it as: There are ships like the Titanic, but you want a reason, what kind of technological improvement would make it possible, that they are used more commonly alongside the airlines instead of dying out? That sounds like just any reason would be okay, because you make a fictional story with an alternate-timeline where you can set the rules as you like. Are the aircrafts supposed to stay as they were or can we also "downgrade airplanes"?

Massive atmospheric turbulence makes flying unsafe.

@Antares they can be downgraded but they can't be gotten rid of

@SurpriseDog "Large jet airliners cannot just be made to simply not exist. Ocean liners should coexist with them. Both should be accessible options for those with the money for international travel."

@OT-64SKOT They exist but their high fatality rate means that half the population refuses to use them.

@GaultDrakkor they should be realistically economically feasible

I'm almost convinced to vote to close the question as Too Story-Based. If you're not asking for a plot point (very off-topic), then you must be asking for a reason that's either (a) technological, (b) economic or (c) cultural. You've explained nothing about your civilization(s), so it can't be (c). I can't imagine a reason why it can be (a). That leaves economic reasons (like jet fuel in your world is outrageously expensive or the economy of scale with ocean liners far exceeds the convenience of quick travel). Can you tell me what the nature of a best answer is?

@JBH the nature of a best answer in this case is something that would allow the world to remain similar to our own in most other respects. The base civilization is close enough to earth that it can be mostly treated like earth.

We are not a brainstorming site. Obviously there is some reason why in your world boats are a more attractive option than planes. It is not our job to build your world for you and generate ideas on your behalf. What have you tried? Why did that not work? Try asking us to help you with that far more specific problem instead of a question with so many equally valid answers.

Concorde, the answer is "Concorde".

@sphennings i do try to answer most of the questions i come up with myself, but sometimes there's something where i just can't figure out a why

No way to "allow the world to remain similar to our own in most other respects" if intercontinental passenger transport is mostly sea borne instead of airborne. Would you have the demand in fuel remain mostly the same, mostly the same pressure on Saudi Arabia to adjust their oil production to the demand, mostly the same pressure on the US govt to enforce the Bush doctrine, mostly the same <insert-your-pet-geostrategic-concern-here>?

@OT-64SKOT I apologize, but that's too vague. Per the help center, we do not help people write stories. Thus, you must judge a best answer based entirely on non-story-centric solutions such as those I mentioned. Story-centric solutions include plot, character decisions, situation and circumstance. Gratefully, most of the answers follow this expectation, but you're obligated to ask based on that expectation.

@DonQuiKong: That wasn't my first thought; my first thought was more "747 Max is the answer.". I mean, if you had to choose between the 747 Max and the Ever Given as your preferred mode of transport....

Though as jokey as my comment above is, the real important question should be - are ocean liners allowed to "drop off the map" and then come back sometime afterwards?

Don’t people still use ocean going ferries for shorter trips of a day or so such as travel from England to Europe?

@JoeW: Yes, also along/across the Mediterranean Sea and in Scandinavia there are plenty of ferry lines. They are cheaper and more efficient and environmentally friendly than planes. Especially once we get around to electrifying them. Battery weight is a problem for planes but not really for ships.

the problem is if you frame it transport over luxury a plane is just overwhelmingly better at transport for ong distance, a ship can only compete via luxury. Its like asking how to mak ice boxes compete with refrigerators when only considering food storage.

You don't have to make turbulence so bad that air travel is unsafe. It just needs to be so bad that most people who can afford to travel by sea (having the time and money to do so) are unwilling to subject themselves to an overseas flight.